,TY Of 'ORNIA TH iNCES LARY MINERALOGY The Q_ THE MACMILLAN COMPANY NEW ^ ORK BOSTON - CHICAGO DALLAS SAN FRANCISCO MACMILLAN & CO., LIMITED LONDON BOMBAY CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, LTD. TORONTO MINERALOGY AN INTRODUCTION TO THE THEORETICAL AND PRACTICAL STUDY OF MINERALS BY ALEXANDER HAMILTON PHILLIPS, D.Sc. PROFESSOR OF MINERALOGY IN PRINCETON UNIVERSITY THE MACMILLAN COM T \NY 1912 All rights reserved EARTH SCIENCES UBRARY COPYRIGHT, 1912, BY THE MACMILLAN COMPANY. Set up and electrotyped. Published October, 1912. NortDoob Ortss J. 8. Cashing Co. Berwick & Smith Co. Norwood, Mass., U.S.A. . ERRATA Page 7, line 14, for 78.2 read - 78.2. Page 9, line 2, for anode read cathode. Page 32, line 9, for 9.815 read 0.815. Page 63, Fig. 107, read Plus Right Tetartohedron. Page 83, line 6, for read ca oa Line 10, for .6494 read .6404. Page 89, line 20, for didigonal read digonal. Page 95, lines 28 and 29, read Mimetite, Pb 5 Cl(As0 4 ) 3 , and Vanadinite, Pb 5 Cl(VO 4 ) 3 . n p Page 112, line 7, for - read c a Page 205, line 19, for cos V = / - read cos V = Page 456, line 34, for ZrO read Zr0 2 . Line 35, for .6493 read .6403. Page 512, line 11, for Pb 5 Cl(PO 4 ) 3 read Pb 5 Cl(V0 4 ) 3 . Page 535, line 17, for 4 Na 2 S0 4 . Na 2 C0 3 read 9Na 2 SO 4 .2Na 2 CO 3 . KC1. 2857^1 PREFACE THE object of this book is to bring together for the beginner, in concise form and under one cover, the facts and basic principles of the several branches of mineralogy, unadulterated by an excess -of data. It is, therefore, not a book of reference, but it takes the stu- dent along the various branches of the subject to a point from which, if he wishes to continue, he will be in a position to appreciate and to use the advanced literature and books on the subject. The system of Dana has been followed to a great extent, as that is the book of reference which is largely used by the American stu- dent, though other sources have been freely consulted in the com- pilation of this volume. In Part I, Crystallography, the thirty-two types have been described for completeness, even though there are no minerals crystallizing in some of them. The names used are those of Miers, as they embody the symmetry of the type and thus require very little memory on the part of the student. Dana's names are given in each case under the term " class." The old method of deriving the hemihedrons, etc., from the holo- hedrons has been retained and may meet with criticism, but it is a simple method of determining what forms are possible to combine on crystals of lower symmetry. I have always found it a very mate- rial aid to the student, leaving no false impressions. Only graphical methods of solving the problems, after the meas- urement of a crystal, are given, and the mathematical solutions are left to the more advanced courses. In Part II, a knowledge of general chemistry is presupposed. Here some two hundred and twenty -five mineral species are included in the general descriptions, embracing the common rock-forming and ore minerals. Their crystallization, optical properties, decomposi- tion products, genesis, occurrence, uses, and synthesis are included in the short description of most of the species. The determinative tables and chemical tests used in the blowpipe table for the identification of the elements are included in Part III. This table includes all minerals with the exception of some very rare species found only in one locality, and in many cases includes even these. It therefore serves the purpose of placing before the vi PREFACE student a nearly complete list of the mineral species with their chemical formula, hardness, color, crystallization, and specific grav- ity. The table has been arranged after years of experience in teaching blowpipe analysis, and only those tests are employed and described which are quite easily manipulated, and wherever possible the dry or blowpipe tests are given the preference. In its scheme it is modeled after Brush and Cornwall's determinative table. The table for the determination of the common minerals by use of their physical properties includes about one hundred and fifty species, and in connection with the short descriptions of these species given in Part II, makes their identification a simple matter. Table I includes about fifty species of the rock-forming min- erals arranged for their identification in rock sections under the microscope. The illustrations, with the exception of two, have been drawn by the author, and the photographs reproduced are of specimens in the collection of Princeton University. For advice and assistance in this work my sincere thanks are due to many of my colleagues, but particularly to my esteemed friend and professor, Henry B. Cornwall. ALEXANDER HAMILTON PHILLIPS. PRINCETON, NEW JERSEY, September, 1912. CONTENTS PART I CRYSTALLOGRAPHY CHAPTER PAGB I. INTRODUCTION 1 II. DRAWING OF CRYSTALS 31 III. ISOMETRIC SYSTEM 47 IV. TETRAGONAL SYSTEM 65 V. HEXAGONAL SYSTEM 84 VI. ORTHORHOMBIC, MONOCLINIC, AND TRICLINIC SYSTEMS . 113 VII. RELATION OF INDIVIDUAL CRYSTALS 134 VIII. ON THE MEASUREMENT OF CRYSTALS AND THE USE OF THE GONIOMETER . . . . . . . . . 149 IX. OPTICAL PROPERTIES OF CRYSTALS 160 PART II DESCRIPTIVE MINERALOGY I. THE RELATION OF MINERALS TO THE ELEMENTS . .219 II. THE ORIGIN OF MINERALS 237 III. PHYSICAL PROPERTIES ........ 256 IV. THE NATIVE ELEMENTS 281 V. SULPHIDES, ARSENIDES, ANTIMONIDES 294 VI. SULPHO COMPOUNDS 320 VII. HALOID COMPOUNDS 327 VIII. OXIDES, INCLUDING THE ALUMINITES, FERRITES, AND CHRO- MITES 337 IX. CARBONATES 379 X. SILICATES AND TITANATES . 403 XI. COLUMBATES, PHOSPHATES, VANADATES, INCLUDING THE NITRATES, BORATES, AND URANATES ..... 507 XII. SULPHATES, CHROMATES, TUNGSTATES, AND MOLYBDATES . 527 viii CONTENTS PART III DETERMINATIVE MINERALOGY CHAPTER PAGE I. DESCRIPTION OF THE INSTRUMENTS, REAGENTS, AND CHEMI- CAL TESTS USED IN THE BLOWPIPE TABLE FOR THE IDEN- TIFICATION OF THE MINERAL SPECIES .... 546 II. TABLE FOR THE DETERMINATION OF THE MORE COMMON MINERALS BY THE USE OF THEIR PHYSICAL PROPERTIES 595 III. TABLE FOR THE DETERMINATION OF THE -PRINCIPAL ROCK- FORMING MINERALS IN SECTIONS 609 IV. TABLE FOR THE DETERMINATIONS OF MINERALS BY THEIR CHEMICAL TESTS . . 617 MINEEALOGY MINERALOGY CHAPTER I CRYSTALLOGRAPHY THE solid, liquid, and gaseous states of matter depend upon temperature and pressure. It is possible to cause a solid to pass to the liquid state by raising the temperature. At C. water passes from the solid ice to the liquid water; C. is the fusing point, or the temperature at which the solid passes over to the liquid water. If the temperature is increased until 100 C. is reached, water passes to the gaseous state steam. The tem- perature at which the vapor passes off freely and where there is no further rise in the temperature of the body of the liquid is known as the boiling point. The fusing point and the boiling point are fixed temperatures for pure chemical compounds. Upon decreas- ing the temperature and increasing the pressure sufficiently all substances become solid. If the solids formed by a slow transi- tion from liquids or gases are examined, it will be found that the larger number are bounded, in part at least, by smooth plane faces. When arsenious oxide is heated it volatilizes; the vapors upon contact with a cold surface condense, forming a white coat- ing ; on examination with a lens, this white coat is found to be composed of small particles bounded by eight triangular faces, Fig. 1. Each individual is a crystal of arsenious oxide. All polyhedra formed by substances when passing to the solid state are crystals. It has been the conception of scientists, since the time of Dalton, that the fundamental unit of matter was the atom; that the num- ber of kinds of atoms is limited, and that each kind possesses dis- tinct properties separating it from all others, thus forming a simple substance, an element. While the number of different kinds of atoms is small, all objects and compounds of nature are possible by the combination of this small number of elements. The number of atoms joining to form the unit or molecule of a compound can, by the law of Avogadro, be determined for all MINERALOGY volatile and dissolved substances. The number of hydrogen atoms combining with one atom of oxygen to form one molecule of water, is two; yielding the chemical expression H 2 for water. The chemical molecule is the smallest particle of a compound which . FIG. 1. Octahedral Crystals of Arsenious Oxide, As 2 O 3 . can exist and still retain the physical properties of the compound. In the attempt to divide the chemical molecule of water, the atoms being indivisible, the one atom of oxygen must be sepa- rated from the two atoms of hydrogen, resulting in two substances, oxygen and hydro*gen, neither of which possesses the properties of water. There is no method of demonstrating whether the solid or crys- talline molecule and the chemical molecule are identical. Since a number of atoms combine to form a chemical unit, it is also probable that a number of chemical units combine to form a crystalline unit or molecule. The study of the structure of crystals indicates that there is such a combination, but as to the exact relation of the one to the other, all that can be said is that the crystalline unit may be expressed by a multiple of the chemical molecule ; in case of ice, n(H 2 0). The light of recent research would indicate that n is a small number, and the crystal unit is not the complex group- ing of a large number of chemical molecules it was formerly thought to be. CRYSTALLOGRAPHY The forces surrounding the molecule in a solid may radiate equally in every direction as if from a center, such a group of molecules would form an amorphous solid ; or again, the lines of force may vary with the direction, when the solid would be crys- talline. The physical properties, as elasticity, hardness, trans- mission of light, conductivity of heat, will be the same for all directions in amorphous solids. Glass, which is an amorphous substance, will expand equally in all directions upon heating. If a sphere of glass is heated and measured at various temperatures it will at each measuring be a true sphere ; while all its diameters have increased in length with the rise in temperature, they all have increased by exactly the same amount. Amorphous solids possess no regular outward form bounded by plane faces, but are ir- regular, globular, or rounded masses, Fig. 2. The physi- cal properties of crystalline solids are the same along parallel directions, but not necessarily so along directions that are not par- allel. The direc- tional variation of the physical properties is a primary character of crystals, and all compounds to be crystalline must possess it ; as will be shown later, this is caused by the regular arrangement of the molecules, assumed as the substance passes from the liquid or gaseous state to the solid. Crystals are generally bounded by plane faces, but the smallest fragment of a crystal will possess this directional quality of its properties, and by it may be identified as crystalline. The tendency to form crystals, or the crystalline force, varies with the substance, being very strong in quartz or calcite, which are almost never found but in the crystalline condition; while in others, as chrysocolla and turquoise, 1 upon which crystal faces have never been FIG. 2. Hyalite from Waltsch, Bohemia : An Amorphous Mineral. Turquoise has lately been found in crystals. 4 MINERALOGY observed, but in which the physical properties are directional, the crystalline force may be considered as being very feeble. The crystalline force will vary not only with the substance, but will differ with the direction in the same substance. When the mole- cules pass to the solid state, from a condition in which they are free to move, and become fixed under the influences of the crystalline force, it is reasonable to suppose that in the direction in which the crystalline force has the strongest attraction the molecules will be packed closer together. In the direction in which the force is feeble they will be farther apart. The forces influencing and surrounding each molecule are exactly the same as the forces influencing and surrounding every other molecule ; there is no sin- gular molecule. The distance from any molecule to its nearest neighbor in the same direction will be exactly equal in all. In Fig. 3, a, b, c, d are 7" in complete crystalline position, while e must be revolved 90, and f must be both revolved and translated to reach a crystalline position in regard to a, b, c, and d. The whole will form a regular molecular network, or point- system, in which each molecule holds an exact position, just as each individual man in a marching regiment must be in his exact position, holding a fixed relation to those surrounding him; and further, if our attention is directed to the complete formation, it will be seen that as a consequence of the orderly position of each man, the whole is bounded by straight lines. If it were possible to place one regiment on top of another, the straight lines would then become planes, and the solid thus formed would be bounded by plane faces ; in a similar way crystals are bounded by plane faces. The smooth plane faces bounding a crystal are its most striking external character. It must always be remembered that they are only reflections of the internal orderly arrangement of the mole- cules. If the crystalline molecules are identical with, or a small multiple of, the chemical molecule, then the crystalline units may be considered infinitely small, as regards any power we may pos- sess to distinguish them when packed together, forming crystal faces. All artificially polished surfaces fall far short of the smooth- CRYSTALLOGRAPHY -*- ness and perfection of the natural polish of crystal faces and cleav- age surfaces. If a crystalline molecule is placed at each corner of a cube, the distance between each molecule will be measured by the length of the edge, Fig. 4 a. The cube may be considered an elementary form or unit of a homogeneous point-system which may be built up, as in Fig. 5. If the cubical unit is w lengthened in one direc- Q b tion, it will now possess FIG. 4. edges of two different values, Fig. 4b. This unit when packed to- gether so as to fill space will produce a regular point-system of another type, Fig. 6. The sec- ond unit may now be broadened, when it will possess edges of three different values, Fig. 4 c, and when packed to- gether will fill space, pro- ducing a point system of still another type. There are fourteen such ele- mentary units, which when packed together will fulfill the crystalline requirements of com- pletely filling space, and place each molecule of the system in such a position that its surroundings shall be exactly the same as the conditions surrounding every other molecule. The shape of the fourteen elementary units must in no way be considered to represent the shape of the molecule, as the space between molecules is far greater than the diameter of the molecule, and there is no method by means of which the shape of a molecule can be determined. These dimensions of space may be considered FIG. 5. FIG. 6. 6 MINERALOGY as representing the sphere of action, or the sphere of vibration, 01 oscillation, of each molecule. While the number of elementary point-systems is limited to 14, the number of complex point-systems must be much extended in order to reach a satisfactory explanation of the symmetry of those crystalline types in each system lower than the normal. These lower types of symmetry may be produced by a combination oJ the elementary point-systems: 1. As if one were pushed within the other, there will be, in such a case, two sets of molecules, one occupying a position in respect to the other, as if translated along a definite direction. 2. By the rotation of one in regard to the other. 3. By both rotation and translation. The number oi point-systems made possible by these methods will reach 230, It still remains true, however, that in regard to their symmetry they will all be included in the 32 possible types of crystals. Definition of a crystal. A crystal is a homogeneous chemical compound bounded by plane faces, and its physical properties are alike along parallel directions. Crystal growth. If growth is considered to be an increase oi size only, then crystals may be said to grow. This crystalline growth must ,not be confounded with organic growth, which is a development. The tissues in an organism increase in complexity the unit in organic growth is the cell, which increases by division, One cell producing two, these in turn increase in size by an assimi- lation of material within the cell wall. Organic growth takes place from within, while crystalline growth goes forward by the attachment of crystalline molecules from without the point- system, extending the individual crystal laterally in every direc- tion by the thickness of each molecular sheet added. One crysta] therefore cannot be an embryo of another, as when a sufficient number of molecules have collected to form a unit of the point- system, and are fixed in the required position, they will possess all the crystalline characters. Microscopic crystals, however small, are just as perfect in regard to their chemical and physical properties as a crystal a foot in diameter, the difference being a mere matter of mass or bulk. Crystallization. It has been shown that in the crystalline state of matter each molecule has a definite and fixed position relative to those surrounding it. In the liquid, gaseous, and dissolved states, every molecule is free to move, and in any direction, leav- ing out of account the so-called liquid crystals; and until their CRYSTALLOGRAPHY 7 discovery, crystals were always considered to be necessarily solids. If it is wished to crystallize any substance, and thus obtain crystals of any compound for study, it will be necessary to bring the substance into one of those conditions of matter in which the molecules are free to move, and then to reverse the process under such conditions that the transition to the solid state will take place very slowly. Each molecule will be fixed upon the network with all equivalent lines of force parallel, providing .always that the substance is capable of forming crystals. Several cases may arise : 1. The substance may be a gas; all gases, with the exception of helium, have been solidified by decreasing the temperature and increasing the pressure. Carbon dioxide, a gas at ordinary tem- peratures, becomes a liquid at 78.2 C.. Liquid carbon dioxide is sold in the market in iron tubes. At ordinary temperatures these tubes are subjected to a pressure of 60 atmospheres. If a small jet of the carbon dioxide be allowed to escape in a beaker, by the sudden expansion and vaporization a large amount of heat is ab- sorbed and the temperature of the remainder caught in the beaker falls below the freezing point, and snowlike crystals of solid carbon dioxide are formed. 2. The substance to be crystallized may be a liquid ; leaving out of consideration supercooling, if the tempera- ture of a liquid is decreased, at a definite temperature, the freez- ing point, crystalline nuclei will appear. From these as centers crystallization will take place until all the liquid has become solid. These centers of crystallization may be seen on the surface of any pool of water just as ice begins to form. 3. The substance to be crystallized is a solid ; substances in this class will fall under three divisions: A. Solids which when heated volatilize without fusion. If the metal arsenic is heated, it volatilizes without fusion ; on resolidification, out of contact with oxygen of the air, the metal will be crystalline. Solids formed in this way are known as sublimates. B. When heated, the substance fuses without chemical change. Such metals as silver, copper, lead, in fact most of the elements, may be crystallized in this way. The crystals of the igneous rocks, as the feldspars, olivine, augite, etc., have been formed from a fusion; only here there has been a segregation, or a separation of the various kinds of molecules at the same time. C. The substance is either infusible, or is decomposed when heated. When a solid is dissolved in a liquid, its molecules pass off from the surface and 8 MINERALOGY move as free as those of a gas. Just as water evaporates in the air, the solid may be said to evaporate in the liquid ; this continues until the liquid is no longer able to hold more of the solid, and equilibrium between the liquid, solid, and the dissolved substance is established, when the solution is said to be saturated. This condition will remain as long as the temperature, pressure, and solubility remain constant. Crystals of copper sulphate may be obtained from a saturated solution by cooling the solution. Salts with few exceptions are more soluble the higher the temperature. Again, crystals may be obtained from a saturated solution by de- creasing the amount of the solvent ; let the solution slowly evapo- rate, both processes will be combined, as the slow evaporation will cool the solution; counteracting this decrease of temperature is the heat of crystallization, for where crystals are forming there heat is being liberated. Perfect crystals may be secured by suspending a small crystal on a thread in the slowly evaporating saturated solution, at the same time guarding against any sudden change in temperature. It is also well to mechanically revolve the growing crystal to insure its being surrounded by solution of the same con- centration, when the deposition will be uniform. Crystals may also be formed from solution by a decrease of the solubility, produced, as in precipitation, by the addition of some reagent in which the dissolved salt is less soluble, or as in the salting out process by the addition of a common ion. All sulphates are insoluble in alcohol ; if alcohol: is poured carefully over the surface of the copper sulphate solution so as to lie as a layer covering the surface, it will mix slowly with the solution and the solubility of the sulphate will be decreased gradually, producing perfect little crystals of copper sulphate. A large number of chemical compounds, especially the more insoluble salts, may be prepared in crystalline form by chemical precipitation. If to a neutral solution of calcium chloride a solu- tion of sodium carbonate is added, a white, fiocculent, amorphous precipitate of calcium carbonate is produced which on standing becomes crystalline. In the first rapid separation the more un- stable amorphous solid is formed, which becomes crystalline, not by the rearrangement of the molecules in the solid, but by a slow trans- fer of molecules from the unstable amorphous solid to the crystal- line nuclei by resolution, the crystalline form being the more stable. The methods mentioned are the more important; there are modifications and combinations of these which are applicable to con- CRYSTALLOGRAPHY 9 crete cases. The metals which are easily reduced electrolytically are deposited on the anode in crystalline form ; these are, however, always distorted and irregular through structural anomalies. With a very weak current good crystals of copper, silver, or lead may be obtained, Fig. 7. Crystallization is a method employed in the separation and purification of chemical compounds, and especially is this so in the commercial field, where efficiency and cheapness are factors of such great importance. Gran- ulated sugar, one of the few chemical compounds produced in enormous quan- tities in almost absolute purity, is sepa- rated by crystallization. mu ": . ii. j FlG - 7. Crystals of Silver The purity of a crystalline compound obtained by Electrolysis, will depend upon the rate of separation, the viscosity of the mother liquid, and its solubility. If perfect crystals are sought, great care must be exercised in the control of the growth of the crystals, the deposition of molecules must go on very slowly. If there is a sudden decrease in temperature of the solution, a heavy shower of molecules upon the forming crys- tals results; they will increase more rapidly along the edges at the expense of the center of the faces, producing skeleton crystals. The hollow faces may ultimately build out, leaving interior cavi- ties filled with mother liquid. All foreign matter incorporated in the body of a crystal, whether of liquid, gas, or solid, is known as an inclusion. The purity of a crystalline salt is inversely propor- tional to the rapidity of formation and to the size of the crystals. When a pure salt is required, it is best to let the crystals form slowly and remove them from the mother liquid while still small. Constancy of angles. The size of a crystal and the general shape will depend to a large extent upon the conditions prevailing at the time of its formation ; the question may be asked, if the size and shape of a crystal is variable, is there anything that is constant upon which the science of crystallography may be based? Nico- laus Steno, a Danish geologist, in 1669, while cutting sections of quartz crystals, noticed that, however variable the outline of the sections may be, due to irregularities of growth and to the difference in size of faces, whenever the sections from the various crystals were cut in a parallel direction the corresponding angles were always equal. The ordinary quartz crystal is terminated by six 10 MINERALOGY triangular faces of equal size, Fig. 8, or by three large faces and three smaller faces, as b, or the point may be stretched out to a straight line, as in c. If a section be cut directly through the apex of a FIG. 8. Distorted Crystals of Quartz, Herkimer County, New York. quartz crystal at right angles to the opposite faces, the outline of the section in every case will be different, but the corresponding angles, Fig. 9, a, or b, are always equal. This law may be stated ~ a, Q, as follows : the dihedral solid angle between simi- lar faces of crystals of the same substance is constant, provided al- ways that the substance is chemically pure and that the angle is measured at the same temperature. Crystal angles are as characteristic of chemi- cally pure compounds as their chemical or physical properties ; not only may they be iden- tified by the angles, but they are an index to the purity of compounds. The constancy of the interfacial angle is a di- rect result of the regular molecular network or point-system. In Fig. 10 a the round dots represent one sheet of molecules of the point- FIG. 9. Parallel Sections of Quartz Crystals. a b FIG. 10. CRYSTALLOGRAPHY 11 system ; should the crystal stop growing after five sheets were laid down it can be seen that the cross section is a square, and the solid formed, a cube. If for any reason growth is irregular and molecules are laid on faster at one end, Fig. 10 b, the cross section is now no longer a square, but a parallelogram ; the solid is no longer a cube, but is elongated in one direction. All angles are right angles and cannot vary as long as the molecules are laid on in this order. We cannot imagine the angles varying from a right angle, any more than it would be possible for a cube to possess angles not right angles. In measuring and comparing angles between similar faces, those faces are considered similar which cut the point-system in the same direction or inclination. The six faces of the cube are, in regard to the point-system, interchangeable. The configuration of the molecules in the plane of each face is the same, therefore the physi- cal properties of each face will be the same and the angles between them will be the same. This is also true for the elongated cube, for the addition of molecules on one side will distort the form, but cannot possibly change the arrangement of those molecules al- ready laid down, upon which the value of the interf acial angles depends. In all similar faces the molecules are the same distance apart in any given direction: they will lie parallel or equally in- clined to the same lines of force ; they will show the same luster, polish, and hardness; they are equally soluble and yield the same corrosion figures ; they will expand with an increase of " temperature equally along parallel di- rections. The distribution of the magnetic force and electric charge will be alike ; in fact, all physical proper- ties of whatever description will be exactly the same, and must be considered in the identification of similar faces. The goniometer. The FIG. 11. The Penfield Card Goniometer. exactness of the interfacial angle is so great that the accuracy of the angles of chemically pure crystals far surpasses the capabilities of any instrument we may 12 MINERALOGY construct, however delicate, to measure them. The more time and patience used in both the construction of the instrument and in measuring the angle, just so much nearer is the result to the theoretical angle of the crystal . The instrument used for measur- ing angles of crystals was invented by Carangeot in 1783. It is to Rome de 1'Isle that the science owes the development of the crys- tal model ; he modeled some 500 forms. It was to facilitate this work that Carangeot, his assistant, devised the contact goni- ometer. This form of instrument is still in use for the rough measurement of large crystals and for crystals with dull faces. molecules in the point-system. This will be understood by a considera- tion of Fig. 6, page 5. In referring to the axes, the verti- cal axis is always denoted by the letter c, Fig. 20. The axis running from right to left in the plane of the paper is denoted by the let- ter b ; the axis running back and front through the paper is the a axis. If in any case the axes are equal and interchangeable, the equal axes are designated by the same letter, as a. If planes are passed through the origin, so that each plane shall con- tain two axes, Fig. 21, there b will be three such axial planes, or principal sections, intersect- ing at a common point o, which will divide space dis- tributed around o into eight octants, in any one of which it will be possible for a crystal face to occur. Each octant is distinguished by measuring in a -f or direction on the axes from the origin o, as indicated in Fig. 20. The upper, front right octant will be -f a, -f b, + c ; the lower, back, left octant will be a, b, c ; the minus sign is the only one written. When the angle between the axes are not right angles, they are distin- guished as in Fig. 21, aob = a, aoc = p, boc = -y. Crystal systems. When referred to their crystaliographical axes, crystals fall into six systems, here defined in terms of their axes. I. Isometric. Includes all those crystals which may be re- ferred to three equal and interchangeable axes at right angles. All three axes are designated by the letter a. II. Tetragonal. Includes all those crystals which may be referred to three axes, all at right angles, two of which, the lateral FIG. 21. Axial Planes and Angles. CRYSTALLOGRAPHY 17 axes, are interchangeable and equal. The axes are designated, a : a : c. III. Hexagonal. Includes all those crystals which may be referred to four axes, three of which are equal and interchangeable, being in the same plane at an angle of 60 with each other ; all are at 90 to the fourth, or c axis. The axes are designated, ai : a 2 : a 3 : c. IV. Orthorhombic. Includes all those crystals which may be referred to three unequal axes, all at right angles. The axes are designated, : b : c. V. Monoclinic. Includes all those crystals which may be referred to three axes, all unequal ; two of these, the lateral axes, are at right angles to each other. One of these is at right angles to the third, or c, axis ; the other is inclined. The axes are desig^ nated, a : : c. VI. Triclinic. Includes all those crystals which may be re- ferred to three axes, all unequal and all inclined. They are desig- nated, & : b : c. Some are accustomed to add to these six systems a seventh system, the rhombohedral or trigonal system, referred to axes parallel to the edges of the rhombohedron. The forms included in this sys- tem are very closely re- lated to the hexagonal system, and can be in- cluded in that system equally as well. Parameters. The distance from the origin at which any plane, or face, cuts a crystallo- graphical axis, Fig. 22, as ob', is the intercept of that plane a'bV on the axis b. This intercept expressed in terms of the unit on that axis, and written as a co- efficient of the symbol FIG. 22. Axial Intercepts, standing for or repre- senting the axis, is the parameter of the plane on the axis. If ob is the unit on the axis b, then ?_, in this case 8, 8b, is the parameter OD 18 MINERALOGY of the plane a'b'c' on the axis b. In the same manner parameters are derived for the axes a and c. In general the parameters of any plane xyz would be ^a: ^b: ^c; oa ob oc in this case 12 a : 3 b : 6 c are the parameters. They definitely fix the inclination to the axes. The actual length of the intercepts varies with the size of the crystal and is unimportant. It is the relative length, one to the other, or their ratio, which determines the inclination of the faces, and fixes the interfacial angles. The plane abc, intersecting all three axes at unit lengths from the origin, is designated by a : b : c, and is crystallographically identical with the plane a'b'c' (8 a : 8 b : 8 c) ; multiplying all the coefficients by 8 simply moves the plane out from the origin parallel to its former position. It still stands with the same inclination to the axes and will intersect all three planes with the same angle as before ; the crystal is only increased in size. It is the custom to simplify the parameters by moving any plane back or forward on the axes until the intercept on one axis is unity. If the parameters 12 a: 3 b : 6 c of the plane xyz are divided by 3, they become 4 a : b : 2 c ; the coefficient of b is reduced to unity. This is the same as moving it to the position x'y'z', cutting the axis b at unity, parallel to the original position. It represents the same crystal in either posi- tion. When a plane is parallel to an axis, it intercepts that axis at infinity, and is expressed oo a ; when a set of parameters contain two infinities, the plane is moved until the remaining intercept is unity and the parameters are written oo a : oo b : c. This system of denoting crystal faces was one of the earliest methods devised, and is known as the parameter system of Weiss ; it has the advantage of simplicity and ^directness in expressing the relation of intercepts which enables one to see at once the relation of the plane to the axes. In the drawing of crystals it is practically necessary to reduce all other symbols to their equivalents in Weiss's system, in order to lay out the axial intercepts ; for this reason it is well to become thoroughly accustomed to the notation of Weiss in the very begin- ning. Indices of Miller. There are a number of other notations which are in use, the most important of which is Miller's system of in-^ dices, now generally used in all works on crystallography. The most general form, or the indices of any plane, are written hkl; the three axes always maintain their usual order. The indices CRYSTALLOGRAPHY 1 9 may be derived from the parameters of Weiss, by dividing the parameters by their least common multiple and reducing the fraction to the lowest terms ; now each coefficient will stand as a fraction in which the numerator is one. Let it be required to convert 2 a : 3 b : 4 c to indices. Dividing by 12, we have T 2 ^ a : T 3 ^ b : T \c, reducing to the lowest terms, ^a : Jb : Jc, the three denomi- nators are then written 643 (read six, four, three) as the indices. The same operation may be expressed thus : the reciprocals of the parameters are written in the order of the axes, cleared of fractions, reduced to their simplest form, and then written as the indices. Taking the same parameters as before, 2 a : 3b: 4c, the reciprocals are J, J, J ; cleared of fractions by multiplying by 12 and reducing to simplest form, the indices 643 are obtained as be- fore. The reverse of this is necessary in order to obtain the param- eters from the indices ; it is almost unnecessary to point out that the indices are always whole numbers and cannot be fractions. When oo appears in the parameters its reciprocal takes its place in the indices. The minus direction on the axes is indicated by writing the sign above the figure, as 123. Examples of equivalent planes : PARAMETERS OF WEISS INDICES OF MILLEB b:c 111 oo b : c 102 2a a ooa a a 2b : - 3c 632 - b : oo c 010 b : 3c 231 b : f c 634 a: oo a IlO Rationality of the indices. All crystals are formed by a regular deposition of sheets of molecules. The relation of these sheets to the point-system of which they form a.part will determine the faces and angles of the crystal, as well as the intercepts on the crystallo- graphical axes. Each intercept is determined by a definite number of whole molecules, for it is impossible to divide a molecule and have it possess the same properties ; when divided it becomes a substance of a different character, belonging possibly to a different crystal system. Every face possible on a crystal is determined by a whole number of molecules which determine the relative size of the inter- cepts. The ratio of the intercepts of any or all planes possible on a crystal to any other plane on the crystal must be a rational 20 MINERALOGY number. Both the parameters and the indices can be expressed in whole numbers, 0, or oo . It has been the experience in the past that, with few exceptions, these numbers are small, rarely larger than 9. In Fig. 23, the sheet of molecules lying in the axial plane cob is represented. Possible planes intersecting this sheet at right angles are represented by aa, dd, ee, etc., each of which intersects *-er FIG. 23. the axis b at greater distances; let all these possible faces be moved up towards o until they intersect the axis c at unit distance, or the diameter of one molecule ; they will now be represented by the dotted lines a'a', d'd', e'e', etc. The ratio of the intercepts of the plane a'a' on the axes c and b is as 1 : 1 ; of d'd', 1 : 2 ; of e'e', 1 : 3 ; of f'f , 1:7; of g'g', 1 : oo . Thus the parameters are all whole num- bers and the ratios are rational quantities. Theoretically it would be possible for a face to occur with an intercept greater than any indicated, but actually they are very rarely observed. The distance between neighboring molecules lying in the plane of any face will increase with the intercept, except when the plane becomes parallel to an axis. The molecules in the plane aa are much nearer each other than the molecules in the plane ee ; mole- cules have the tendency to crowd together as closely as possible. It follows therefore that those faces will appear the more often on crys- tals in which the molecules are the nearest. In Fig. 23, the cube face hh and rhombic dodecahedron aa will occur the more often, as CRYSTALLOGRAPHY 21 FIG. 24. the molecules are the nearest in the planes. They will also possess small intercepts and a simpler relation of their parameters. Crystal forms. When space is divided by the three axial planes into eight octants it is evident that one set of parameters may rep- resent more than one face ; in fact there will be eight planes, or one in each octant. The solid bounded by these eight faces is known as a crystal form ; the symmetry of the type may not, however, require all the eight faces to be present. A crystal form may be denned as the solid bounded by the combination of all those faces possible to be represented by one set of parameters irrespective of sign ; and required by the symmetry of the type. The combination of planes may inclose space or may not. In Fig. 24, eight faces are shown, all of which are represented by the set of param- eters, a : b : c, and the symmetry of the type requires all eight faces to be pres- ent ; Fig. 25 represents four of the same faces, but producing an entirely differ- ent form, as the symmetry of the type requires only four of the eight faces to be present. When one faee only is in- tended to be represented by a set of parameters, or indices, they are written without pa- renthesis 111 ; when the entire form is represented, they are written (111). The number of faces possible on any crystal form may vary from 48, in the type which possesses the highest sym- metry, to one face in the type which con- tains no symmetry. As at least four faces are required to inclose space, there are two classes of crystal forms, those that inclose space and are termed closed forms, Fig. 26, and those which do not in- close space, and are termed open forms, Fig. 27 ; theoretically the open forms extend to infinity on the open sides, unless terminated FIG. 25. FIG. 26. Unit Pyramid of Barite ; a Closed Form. 22 MINERALOGY by a combination with other forms. Combinations of open forms may inclose space. The number of faces occurring on any crys- tal form is very limited, but the number of faces possible on a crystal which is a com- ! i . -f - j i FIG. 27. An Open Form. FIG. 28. Combination of a Closed and Open Form. bination of crystal forms is not limited and in some cases may be very large. The forms which may occur in combination on crystals are limited to those possible to be derived from the same point-system, and they will therefore have the same symmetry. From the sym- metry of the type, forms in combination always bear the same relation to each other; Fig. 28 is a combination of the closed form of Fig. 26 and the open form of Fig. 27 ; here equivalent edges are cut by the prism, or the four edges of the pyramid are replaced by the prism faces. When the replacement is symmetrical, as in this case, the angles between the prism and the pyramid faces above and below are equal ; the pyramid edges are said to be truncated by the faces of the prism. In the same way, corners of forms may be truncated by other forms and replaced not only by one face, but by a group of faces, Fig. 29. When the edge of one form is sym- metrically replaced by two faces, it is said to be beveled, Fig. 30. Zones. The edge of a form may be replaced by a series of faces, the mutual intersections of which are all parallel to the edge replaced. Such a series of faces is termed a zone. The intersections of all faces possible in any one zone will be rep- resented by possible edges on the crystal, all parallel to each other and parallel to an FIG. 30. The Cube bev- . . ,. . eled by the Tetrahexa- imaginary line drawn through the point of hedron. intersection of the crystal axes, termed the FIG. 29. The Cube with the Corners replaced by the Tetragonal Trisocta- hedron. CRYSTALLOGRAPHY 23 zonal axis, Fig. 31. In the study of crystal faces it will be found that they all belong to a comparatively few zones. The intersec- tion of any two faces on a crystal will determine the direction of a pos- sible zonal axis. Faces belonging to the same zone must be so related that two of their inter- cepts will bear a con- stant relation, and their intersections with the axial plane in which these two intercepts are measured will be paral- FIG. 31. Crystal of Topaz in which the Faces c, i, u, o, e, and m are in the Same Zone, the Axis of which is aa'. lei lines. In Fig. 32 four faces belonging to the same zone are rep- resented and extended to the axes a and b ; the ratio of these in- tercepts is easily understood from the similar triangles, and the intersections of all the faces with the axial plane aob are parallel lines. A zone may be interrupted at any point by the interposition of other faces not belonging to that zone. Zonal re- lations help very materially in the measurement of crystals, for once a face has been located as a member of a zone, its parameters when determined must fulfill the zonal relations. Fundamental forms. Among the faces found on the crystals of any substance, a face which cuts all three axes, and is simply related to all other faces occurring on the crystals, is selected ; its intercept on each axis is taken as the unit of measurement on that axis ; its parameters would be a : b : c ; the form is termed the FIG. 32. 24 MINERALOGY unit, or fundamental form, to which all other faces are referred. The intercepts of the unit form on all interchangeable axes are equal, their ratio - = 1 ; the intercepts on axes that are not inter- changeable are always an indeterminate quantity, 7- = 0.81520+ ; -= 1. 31359 +, when b is taken as unity and express the ratio of b the units on the axes. The axial ratios of barite are written & : 5 : c = 0.81520:1:1.31359. For chemically pure substances the axial ratios are constant and are characteristic of the substance, just as much as any of its chemical properties. The axial ratios and the value of the interaxial angles in the monoclinic and triclinic systems, which are also constant for pure substances, are termed the crystalline characters or elements. The crystalline characters in the isometric system are determined by all the axes being inter- changeable; they are the same for all substances that crystal- a c c lize in the system; - = 1. In the tetragonal system, - = y = a a 1 0.1644154 +, axial ratio of rutile. In the hexagonal system c c - = j- = 9.734603 +, apatite. In the orthorhombic system there are two axial ratios, ^ and ^, a : B : c = 0.81520+ : 1 : 1.31359 +, axial ratios of barite. In the mono- clinic system the two axial ratios and the value of the angle p are he crystalline characters; a : b : c = 0.658510 + : 1 : 0.55538 +, P = 63 56' 46", orthoclase. In the triclinic system there are three angles in addition to the axial ratios : & : b : c = 0.49211 + : 1 : 0.47970+ ; a = 82 54' 13"; p = 91 51' 53"; Y = 131 32' 19", axinite. Holohedral, holosymmetric, or normal, are terms denoting a type of crystals in each system, in which the symmetry requires all the faces possible to be repre- sented by one set of parameters to be present to complete the FIG. 33. Holohedral Form a : c : 3 a, (331). CRYSTALLOGRAPHY 25 form. The set of parameters a : a : 3 a represents three faces in each octant, as here the axes are interchangeable and when one axis is cut all must be cut by a plane at 3 ; these three planes are rep- resented by (a : a : 3 a), (a : 3 a : a), (3a:a:a), or 24 faces in the eight octants; Fig. 33 repre- sents this form. Hemihedral form is the term used to denote those types in which the symmetry requires only one half of the faces pos- sible to be represented by one set of parameters, to be present to complete the form. There are several classes of hemihe- drons, according to their symmetry. If the most general form of a system as (a : 2 a : 3 a), represented by Fig. 34, with 48 faces, is taken, there may be numerous ways of selecting one half of these 48 faces; the symmetry of the types allows but three to form hemihedrons. I. By taking all the faces in alternate octants and extending them until they inclose space, as the shaded faces of Fig. 35, which when FIG. 34. The Hexoctahedron, a : 2 a : 2 a. FIG. 35. FIG. 36. extended will produce the form represented in Fig. 36. Since some holohedrons have a center of symmetry and are formed of pairs of parallel faces ; this class of hemihedrons in which one face of each pair of faces of the holohedron is extended, will not be formed of 26 MINERALOGY pairs of parallel faces, and for that reason they are known as the diagonal-faced hemihedrons, represented in the Miller system of indices by ic(hkl). II. By selecting one half of the pairs of faces, taking those which intersect in the axial planes, as represented in Fig. 37'; these when FIG. 37. FIG. 38. extended will produce the hemihedral class, Fig. 38, known as the parallel-faced hemihedrons, designated in the Miller system by III. By selecting every other face around the extremity of an axis, and those alternating with it around the adjacent axis, as repre- f sented by the shaded faces in Fig. 39 ; these when extended will FIG. 39. FIG. 40. produce the gyroidal, or plagiohedral, class of hemihedrons, repre- sented in Fig. 40. These are denoted in Miller's system by r(hkl). CRYSTALLOGRAPHY 27 In all cases there are two forms of hemihedrons possible to be derived from each holohedron, for the white faces in each case could be extended to obliterate the shaded faces ; one of these is the +, the other the hemihedron, in classes I and II. In class III they are right and left forms. Tetartohedral forms. In some types of crystals with still lower symmetry, only one quarter of the face of the general form may be required by the symmetry to complete the form; such forms are termed tetartohedrons. The faces extended to form te- tartohedrons must in each case modify the extremities of inter- changeable axes in the same manner. If in Fig. 41 the shaded faces are extended, the tetartohedral form of Fig. 42 will be produced, having 12 faces. This is the right positive form, designated + R ^-~ or irk (hkl). If the three unshaded faces in the upper right octant and the corresponding faces in other octants are extended, the -f- left form will be produced. These two' forms are FIG. 41. (a:2a:3a.) FIG. 42. + R mirror images of each other ; there is no way in which they can be revolved into congruent positions. It is like a left glove on a right hand; such forms are enantiomorphic. Two other forms are possible: the minus right produced by extending the faces - R, Fig. 41, and the minus left produced by extending - L. -There are always four tetartohedral forms possible, the rights and the lefts ; the rights are congruent with each other and the lefts are also congruent, but the rights are enantiomorphic with the lefts. 28 MINERALOGY Models. In the study of crystals and the relation of crystal forms, models cut from wood are indispensable. The student should cut the simpler forms and their combinations from cork, as rela- tions once established in this way are never forgotten. Crystal models are cut showing similar faces of the same size, or equally developed, thus representing the ideal symmetry of crystals, Fig. 43. In nature crystals seldom if ever present the ideal symmetry, as some faces are always enlarged at the expense of others ; in this way some faces may be entirely obliterated, when the appearance of a crystal may be so changed by the unequal development, or distortion, as to be difficult of recognition. Distortions take the form of elongation along a set of parallel edges, as in Fig. 44, a FIG. 43. FIG. 44. A Distorted Rhombic Dodecahedron of Garnet. distorted rhombic dodecahedron of garnet with the edges parallel to ab elongated ; again, in the distortion of crystals, points are re- placed by edges, as in Fig. 45 a and 45 b, a regular crystal of quartz CRYSTALLOGRAPHY 29 and a distorted crystal in which parallel edges c and d replace the points c and d, and the edge ab is elongated. Habit. Different combinations of the various forms in which a substance may crystallize will produce crystals of widely varying FIG. 45 a. Symmetrical Quartz Crystal. shapes, and especially when combined with distortions. These com- binations, peculiar to localities or conditions of crystallization, are known as the habit. Forms will be found on crystals from one c FIG. 45 6. A Distorted Quartz. locality which may not necessarily be found on those from another. Even though the forms are identical, their relative development will 30 MINERALOGY yield a crystal of an entirely different appearance. Figure 46 is a crystal of barite from Felsobanya with a tabular habit; Fig. 47 is a crystal of barite from Cumberland, Eng- land, with a prismatic habit, being elongated parallel to the b axis ; the two crystals present combinations of the same forms ; their different appearance is due to inequality of development. FIG. 46. Tabular Habit of Barite from Felsobanya. FIG. 47. Elongated Habit of Barite ; a Combination of the Same Forms as in Fig. 46. CHAPTER II CRYSTALLOGRAPHY Drawing of crystals. The object to be attained in the drawing of crystals may be either to represent their relation and habit in perspective, or to represent the relation of forms on individual crys- tals. The methods in use for this purpose are those of general projection, though modified in some cases to fit the conditions. The edges of crystals are formed in every case by the intersection of two faces ; in the drawing they are represented by straight lines ; to find the position, inclination, and foreshortening of any edge is nothing more than a problem in the intersection of planes. The position of each face is given by the crystallographical symbols. Interfacial angles are used in the drawing, only in so far as they are necessary to determine the axial ratio and intercepts. The methods of perspective projection are modified, not only to simplify the construction, but to adapt it to the representation of crystal edges, so that edges parallel on the crystal will be parallel in the drawing, and of a length proportional to their actual length on the crystal. This modification is simply placing the eye at infinity ; all rays will then be parallel. All parallel and zonal directions will be preserved ; it will be necessary to determine the direction of only one edge of a zone in the drawing, all other edges in the same zone will be parallel. It is customary to represent the ideal symmetry of a crystal in the drawing, unless it is wished to illustrate some peculiar development or habit. There are two general methods of projection, the orthographic 'and clinographic methods, both of which place the eye at infinity. Orthographic projection is a plan or map of the crystal faces and edges, drawn on a plane perpendicular to the c axis. It represents in crystallography exactly what the foundation and roof plans of a house do to a builder. When the plane of projection is perpendicu- lar to the c axis the eye will lie in the direction of the c axis, at an infinite distance, vertically above the plane ; the c axis will appear 31 32 MINERALOGY m as a point in the center of the drawing at the intersection of the lateral axes. All edges parallel to c will also appear as points. Planes parallel to the c axis will appear as straight lines. The angles between these planes will be represented of true size. . Edges parallel to the plane of projection will also be represented in the drawing by lines equal to the lengths of the edges ; edges inclined to the plane will be proportional to their inclination. Supposing it is wished to represent the crystallization of barite, axial ratio, : b : c = 9.815 + : 1 : 1.313 + , and the following forms: a = (100) = a: oob :_ooc, b = (010) = oo^:b^ooc, c = (001) = oo a : oo b : c, d= (102) = 2 a : oo b : c, m= (110) = a:b : ooc, p= (111) = a : b : c, When the indices alone are given it is necessary to transform them to Weiss parameters, as these represent the proportional inter- cepts on each axis. As the two lateral axes appear in the draw- ing in their true length, lay out from c, Fig. 48, the vertical axis, a distance cb and cb' each equal to b, the selected unit of length; at right angles to cb', as barite is an orthorhombic mineral, lay off ca and ca', each = cb X .815 ; through b,b' draw lines parallel to aa', which will represent the form (oio) ; through a, a' draw lines par- allel to bb' representing the form (100) ; m (no) would be rep- resented by joining the extremities of the axes a and b, but the cross section can be varied to suit the crystal at hand, by moving the line connecting a, b out to a parallel position m, when m, m', m", m'" drawn symmetrically will represent the four faces of the unit prism, m (no) ; these four lines* will also represent the inter- section of the unit pyramid, p (in), with m (no). The apex of the pyramid is at c and the ridges will fall on the axes a and b. The base c (ooi) is in the same zone as m (no) and p (in), therefore the intersection of c and p will be parallel to the lines m, m', rep- resented by four lines drawn around c parallel to m, m', m", m'". The size of the base can be varied by the distance from c at which the four lines are drawn. Of the forms to be represented there FIG. 48. Orthographic of Barite. CRYSTALLOGRAPHY 33 remains the dome, d (102) ; a construction section of the crystal, containing the a and c axes, must be drawn, Fig. 48 a ; from b draw be at right angles to the base. Make be = b (the unit on b) X 1.313, and ba = b X .815, connect c and a which will represent the ridge of the unit pyramid p ( 1 1 1) . The base c cuts this pyramid at a point p, found by laying off bo equal to cc', Fig. 48, projecting up to the ridge of the pyramid at p, then pc will be the base at the same height at which it is represented in Fig. 48. The form (102) will cut the axis at a', ba' = b X .815 X 2, and the c axis at c, be = b X 1.3-13 ; ca' connecting these two points will give the slope FIQ ' and angle of the macrodome (102), this may be moved in parallel to ca', to cut the pyramid at any required point as at cd; project cd to cd' and lay off on the a axis in Fig. 48 from c, ex, and cy, also ex' and cy' = bd' and be', then yy' will be the projections of the points at which the dome enters the edge of the pyramid; through xx' draw lines parallel to the b axis,, which will be the intersection of the dome with the base ; then the triangle xzy, and similarly above x'z'y', will repre- sent the dome faces. Uniformity of lettering. It is the custom to adopt a uniform system of lettering crystal forms, at least those forms which deter- mine the crystalline characters, that they may be recognized' at once and the position of the axes fixed. The pinacoid (100) = a; (010) = b; (100) = c; m represents the unit prism (110); p, the unit pyramid (111). In the hexagonal and tetragonal systems the prism of the first order is m, that of the second order a, and the unit rhombohedron is r. In the isometric system a, o, and d represent the cube, octahe- dron, and rhombic dodecahedron respectively. In addition to the above, individual faces are indicated by accents ; all faces in the right front octant are not accented, faces in the right back octant are indicated with one accent, etc.; thus p, p', p", p'" would indicate the four upper faces of the unit pyramid. Clinographic projection. The clinographic method of illustra- tion, in addition to expressing the relation of various forms, also gives the impression of solidity and perspective, which is not the 34 MINERALOGY object of the orthographic method. The disadvantage of the method is that no angles and only those edges which are parallel to the c axis are given their true size in the drawing ; one method therefore supplements the other and crystals should be illustrated by both methods. In the clinographic method the crystal is projected upon a ver- tical plane containing the c axis. The c axis is the only one given its true length in pro- jection. In Fig. 49 an octahedron is rep- resented in ortho- graphic projection; in order to pass to the clinographic two steps are necessary : 1st, the crystal is revolved around the c axis some selected angle, usually 18 26', after this revo- lution the octahedron will assume the posi- tion of the dotted lines. The axis oa will be moved to oa' ; when projected upon the plane of projection of which xy is the trace it will appear foreshort- ened as oa" ; in the same manner oai will appear upon the plane of projection as oa/ ; the amount of foreshortening will depend upon the angle of revolution. 2d, after -the revolution about the c axis, in order that the plane of the lateral axes shall not be repre- sented in the drawing, by a line, as xy, the eye, hitherto at infinity in a horizontal direction, is now elevated until the lines of vision form an angle, selected generally as 9 28', Fig. 50, with the horizontal plane. It is as if the eye, being at the same height above the floor as the table top, the relative positions of objects on the table would not be appreciated, as the lines of vision are parallel to the table top ; if the eye is elevated, the top will come immediately into FIG. 49. CRYSTALLOGRAPHY 35 9 ae c FIG. 50. view and the relative positions of objects on the table is at once seen. A clinographical projection of an octahedron is represented in Fig. 49, in which the projection of the plane of the lateral axes aa'a is the result of > f\ an elevation of the eye 9 28'. Referring to Fig. 50, cc' is the trace of the plane of projec- tion; it will be seen that every point on the horizontal plane in a, -"" front of the plane cc' will appear below o, and every point behind cc' will appear above o; the distance above or below o at which any point will appear de- pends upon the angle of elevation of the eye and the distance from o of the point in question. Take any point a, Fig. 50, the line of vision aa' is 9 28' from the horizontal, and a will appear on the plane of projection at a'. In the triangle aoa', where oa = 1, oa' = the tangent of oaa' = tangent 9 28' = | oa ; the point ai will appear at a/, | of aio above o ; the point e will appear at e', | oe below o, etc. When the angle of elevation is 9 28', the distance of any point from the plane of projection, measured below o if the point is in front and measured above o if the point is behind the plane of projection, will deter- mine the projection of a" -----i/ the point in question. FIG, 51. The Axial Cross of the Isometric Construction of the System. axial crosses. Unless to illustrate some peculiar conditions the angles 18 26' and 9 28' have proven the most satisfactory and are in general use. 36 .MINERALOGY -C FIG. 52. Axial Cross of Zircon ; c = 0.64+ . Isometric. Draw xy, Fig. 51, the trace of the horizontal plane with the plane of projection, and a'a' at right angles, making o a', ao = the true length of the axis oa. Draw oa' at 18 26' to xy, making oa' = oa; draw a'a" at right angles to xy; as a' is in front of xy lay off a distance a"a'" = i a'a", draw oa"' and extend it to a'"o, making a'"o = a'"o ; then - a"V" will be the projection of the axis. The axis at right angles to this is gotten by drawing oai at right angles and equal to oa' ; draw a'iai per- pendicular to xy and make a/'a/ = -J a/ai; draw a/'o, extend to - a/', making -- a/'o = oai", when a/'a/' will be the projec- tion and relative length of the second lateral axis. The three lengths a'"o, a/'o, a'o all rep- resent the same unit of length as measured in turn on each of the FlG - 52 a. The Unit Pyramid of i .. . ! . Zircon ; c = 0.640. axes ; by connecting the extremi- ties of the three axes the clinographical projection of the unit form or octahedron will be obtained, Fig. 49. Tetragonal. The tet- ragonal system will not differ from the construc- tion in the isometric, ex- cept that the c axis is not equal to the lateral axes. Draw xy, Fig. 52, and find the projections of the two lateral axes exactly as described in the isometric cross ; at o draw oc = co = oa X (the axial ratio of the mineral to be represented), in FIG. 53. Axial Cross of Apatite ; c = 0.734+. CRYSTALLOGRAPHY 37 zircon .640. The unit pyramid of zircon will be projected by connecting the three axial units, Fig. 52 a. Hexagonal. Here the problem of the axial cross differs, as the lateral axes are three and not at right angles. Draw xy, Fig. 53, and project c and a 2 as before. Draw a 2 'oa 3 ' = 60, making a 3 'o = a 2 'o, then draw a' 3 a 3 par- allel to cc, placing a 3 , | the distance from xy as a' 3 , when - a 3 a 3 will be the projection of the axis required. The third lateral axis is found by laying off the angle a/oa/' = 60 and following the same construction as before ; by connecting the ex- FIG. 54. The Unit Pyramid of Apatite. tremities of all the axes the unit pyramid will result, Fig. 54. Orthorhombic. Here the three axes are at right angles, but all of a different length. Taking barite as an example, where a : b : c = .81 + : 1 : 1.313, to project the axial 'cross. Draw xy, Fig. 55; lay out ob' at 18 26' from y and some selected unit in length as 50 mm. ; find the pro- jection of b as before. The c axis is found as in the tetragonal system, oc' = 50 mm. X 1.313+ = 65. 65 mm. The a axis is found by laying off oa' at 90 to ob', and = 50 mm. X. 81 =40.5 mm., when the projection aa is obtained as before; connecting the extremi- ties and Fig. 56 will rep- resent the unit pyramid of barite. Monoclinic. Here the problem differs in that the clinoaxis a is not at 90 to the vertical axis c. Let it be required to draw the axial cross of amphibole, a : b : c = .55+ : 1 : .29 + , 'ft = 73 58'. Project b and c as in the orthorhombic system; to find the pro- FIG. 55. Axial Cross of Barite. 38 MINERALOGY jection of a it is necessary to understand the position of the angle ft ; the position of ft is always back, Fig. 57, of the plane of pro- jection, here represented by cc; its value is always taken less than 90. Therefore the angle be- tween the c and a axes in front will be greater than 90, in this case 106 2', and the extremity of the a axis in front will be be- low the horizontal plane a dis- tance oa"', depending upon the value of the angle ft and the length of the a axis. This dis- tance is found by drawing aoc = ft and extending in the direc- tion of a, making oa = ofe X 9-55 + ; draw aa'" at 9 28' with the hori- zontal, then the projection of the end of the clinoaxis in front will FIG. 56. The Unit Pyramid of Barite. appear below xy a distance oa'", due to the angle ft -f a"a'" (i aa") due to the angle 9 28'. Therefore lay out in Fig. 58 the angle aoa' = 16 2' and oa' = the unit on the a axis, then the extremity of the. a axis will appear below xy a dis- tance aa' due to the angle ft, then by the revolution of 18 26' and the elevation of the eye 9 28', the projection of the extremity will fall under a'", a distance aV" = aa' + | a"a'"; connect a with o and extend to - a an equal distance, when a will be the projection of the clino- axis; connecting the extremities of the projected axes, when Fig. 59 will represent the unit pyramid of amphibole. Triclinic. Let it be FIG. 58. Axial Cross of Amphibole. required to draw the CRYSTALLOGRAPHY 39 axial cross of rhodonite, a : b : c = 3.072 + : 1 : .6212 + ; = 193 18' ; (3 = 108 44' ; y = 81 39' ; and the angle 100 A 010 = 94 26'. The construction of the c and & axes is the same as in the mono- clinic system, here the plane containing the b and c axes is not at 90 from that containing the a and c axes, but as in this case is 94 26'. Having constructed the projections of a and c, lay off the angle a"ob' = lOOvOlO = 94 26', make bob' = a - 90 = 13 18' ; make ob the true length of b, draw bb' at 90 to ob'. extremity of the b axis will lie below the horizontal plane a dis- tance bb', due to the angle being 13 18' larger than a right angle ; draw b'b" at 90 to xy; the projection of b will lie on the line FIG. 59. The Unit Pyramid of Am- phibole. The FIG. 60. Axial Cross of Rhodonite. FIG. 61. b'b", extended, if necessary, a distance b"b = bb' + J b'b", b b will then be the projection of the axis b. Figure 61 represents the combination of (100); (010), (001), (110) of rhodonite. Example I of clinographic projection. After the axial cross is projected, the clinographic projection of any crystal is a problem in the intersections of planes ; the inclination of the faces is given by the parameters. Let it be required to project clinographically the same forms used to illustrate the orthographic method on page 32. Construct the axial cross and connect the extremities of the axes, Fig. 62, which will represent the unit pyramid (111) ; the base, c = (001), will truncate the pyramid above and below o, and will be parallel to the plane containing the axes a and b. Let it cut the c axis above at c, below at ci. This distance will depend upon 40 MINERALOGY the development of the crystal to be illustrated, as all faces may be moved back and forth on any axis, if their inclination be not changed. The intersection of c with the edge of the pyramid is found by drawing c'cc" parallel to the axis ; where this line inter- sects the edge of the pyramid at c' will be a point common to both the edge of the pyramid and the base ; likewise c", the intersec- tion of c and p, will be parallel to pp' as the two edges belong to the CRYSTALLOGRAPHY 41 same zone ; draw c'c'" parallel to pp', c'"c" parallel to p'p", and so on around the four sides of the base above and below. The unit prism, m (110), is a member of the same zone, and its intersection with the pyramid will be parallel to the intersections of the base and pyra- mid. If the prism cuts the b axis at m, then m'm" drawn through m parallel to oc will represent its edge in projection, and m', m" will be points on the intersection of the prism and pyramid; draw m'm'" parallel to p'"p, and so on around the pyramid above and below, dotting edges which will appear behind. To project the remaining form d (102) = 2 a: oob : c, lay off on the & axis, 2 a, connect 2 a with c and c ; these lines will be the intersection of the dome with the axial plane coa. Move 2 ac in parallel to itself, until it cuts the edge of the pyramid at d"' and the base at e, the inter- FIG. 63. FIG. 64. One Octant of 3 a : 3 a : a. section of d and c will be parallel to the b axis, as both faces are parallel to b ; draw ee" parallel to p'p'" and connect e"d'", e"'d'", 42 MINERALOGY when e"d'"e'" will represent one of the dome faces; the remain- ing three are projected in a similar way. Figure 63 represents this combination without the construction lines. Example II. Let it be required to draw (113) = 3 a : 3 a : a of the isometric system. Draw the axial cross and extend them in each case to 3 a, Fig. 64 ; in the isometric system all the axes are inter- changeable, and where one axis is cut by planes at 1 and 3 the other axes must be cut by two planes also; there will be three planes in each octant ; connect 3 a, a", 3 a', the three lines will be the inter- section of the face 3 a : a : 3 a with the three axial planes separating a. the octants; likewise connect 3 a', a, and 3 a" ; 3 a", 3 a, a'. The intersections of these three planes are 3 ac, 3 a"ad, 3a'b; those portions of the intersections which represent crystal edges are drawn in full lines. The portion of the crystal represented is that contained in one octant or one eighth of the whole form. All edges in the three remaining octants in front of the plane of projection are obtained by the same method of projection. Those in the four octants behind the plane of pro- jection, where the form is symmetrical in respect to a center, may be conveniently drawn as in Fig. 65. If from any point c a line be drawn to the center o and extended beyond the center an equal distance as co, c will be the point behind the plane of projection corresponding to c in front, likewise all other points may be located by drawing lines through the center o and the faces represented by connecting these points. Spherical or stere graphical projection. In the spherical pro- jection the crystal to be represented is placed at the center of a sphere so that the intersection of the crystallographical axes coin- cides with the center. The plane of projection passes through the center of the sphere at right angles to the c axis, and intersects the sphere in a great circle, the equator ; the c axis when extended will intersect the sphere of projection at the north and south poles. The eye is situated at the south pole to view the faces located in -b FIG. 65. CRYSTALLOGRAPHY 43 the northern hemisphere and at north pole to view those in the southern hemisphere. Crystal faces are not represented by the pro- jection of their edges, but by the location on the plane of projec- tion of the point of contact with the sphere of a radius perpen- dicular (the normal) to the face, as viewed from the south pole. In Fig. 66 the plane of projection per- pendicular to the paper is repre- sented by bb', which also represents the b axis; the c axis c'e ; the a axis perpendicular to the paper will be represented by a point at o. Four faces c, f, d, b, belonging to the zone of which the axis a is the zonal axis, their normals when ex- tended will intersect the sphere at their respective poles c', f, d', b'; these poles when viewed by the eye at e will appear on the plane of projection, bb', as if they were actually located at o, i" , d", b', their projections. The distance from o at which the pole of any face will appear in the projection, as f, is proportional to the tan- gent of one half the angle foe ; for of" = oe tan oef " ; oef " = \ cof ' ; tan 26(foc = 26) = .23, or the distance of f" from o is 23/160 of the radius. It may be seen by the construction of Fig. 66, that all the normals of any one zone will lie in one plane ; their poles will all lie on the great circle in which this plane intersects the sphere of projection ; therefore the arc between any two poles is a measure of the angle between their normals. It is the supplement of the angle between the crystal faces which the normals represent. The arc f'd' measures the angle d'of', and as the angle of a = oda = 90, then d'of -f- daf = 180. As the angle between the normals is the angle actually measured by the reflecting goniometer, it is the angle reported and used in the descriptions of crystals. All poles in the northern hemisphere when viewed from the south pole will fall upon the plane of projection within the equator or primitive circle. When the plane of projection is a plane of sym- metry, the projection of the northern hemisphere will also be a pro- jection of the southern hemisphere as viewed from the north pole. Similar poles above and below will coincide on the plane of pro- jection. All zones or great circles perpendicular to the plane of projection are projected in diameters of the primitive circle; all 44 MINERALOGY zones or great circles inclined to the plane of projection are pro- jected in arcs of circles cutting the primitive circle at the ends of a diameter. Before illustrating by an example the method of drawing the spherical projection of a crystal, it is necessary to have well in mind several problems constantly in use during the con- struction. If from the pole of a zone circle, lines be drawn through the poles of any two faces lying upon the zonal circle, and extended until they intersect the primitive circle, the arc of the primitive circle inter- sected will measure the angle between the normals of the faces. Problem I. Given the projection of any great or zone circle, to find the projection of its pole. Let dsc, Fig. 67, be the zone circle ; draw the diameter dc, and the diameter so perpendicular to dc, so will be the projection of a great circle, with its pole at c and at 90 to dsc; therefore the pole of dsc must lie on so 90 from s ; draw cs and extend to intersect the primitive circle at s, lay off sa = 90, connect ac, and where it crosses os at p will be a point on a great circle at right angles to the given zone dsc and 90 from it ; it is therefore the pole of dsc. Problem II. Given the projection of the pole of any zonal circle, to draw the projection of the circle, is simply the reverse of problem I, Fig. 67. Problem III. Through any two given poles to draw the projection of the great or zonal circle to which they belong. In Fig. 68 let P, S be the d two given poles ; then draw the diameter po, and oa at 90 to it, then a is the pole of the great circle of which Po is the projection and the FlG ' 6a given pole^ P will lie upon it ; draw aP, extend it to P', lay off P'b = 180; draw ab to meet Po extended at B; B will be the FIG. 67. CRYSTALLOGRAPHY 45 FIG. projection of a point on the sphere at the opposite end of a diame- ter from P ; therefore draw a circle through BSP, and cSd will be the projection of the zonal circle re- quired. Problem IV. Given the projec- tion of any two poles, to find the angle between them. In Fig. 69 let a, b be the given poles; by Problem III draw the zonal circle dabc, and find its pole P by Problem I ; draw Pa and Pb, extending them to meet the primitive at a'b', then the arc a'b' will measure the angle between the normals ab. Problem V. Given the zone circle and the projection of one pole in the zone, to find the projection of a face in the same zone at a given angle from it. This is the reverse of III. In Fig. 69, let dac be the zone circle, a the given pole, to find the pole b, 80 from it. Find the pole p of dac by I; draw Paa' and make a'b' = 80 and draw b'P; where it crosses the zonal circle dac as at b will be the projection of the pole 80 from a as required. Problem VI. To locate the pro- jection of any pole, the axial ratio and the indices of the face being given. The axial ratio of barite is & : b : c = .815 : i : 1.313 ; locate the pole of y = (122) = 2 a : b : c. In Fig. 70 draw the primitive and the two diameters aa', bb' at 90. Lay off ob" = b, the intercept on the axis a is oa" = 2 (b X .815) ; draw b"a" and oc at 90 to a"b", then oc will be the projection of a zonal circle at 90 to the primi- tive with its pole at P, on which the pole of 122 will lie. Lay off oc' = b X 1.313, or the unit on the vertical axis, and od = oc, connect d and c' ; the angle c'do = the angle between the normal of y, (122) and the normal to the base ooi ; draw Poc", and make c"oy' = c'do, draw y'P ; where it crosses oc as at y will be the projection of the pole of 122 as required. FIG. 70. 46 MINERALOGY FIG. 71. Example. Let it be required to project a barite crystal with the following forms: (100) (010) (001) (110) (102) (Oil) (111) (112) : 1HU10 = 78 22' : OOU02 = 38 51' : 001,011 = 52 43': 001*112 = 45. Draw, Fig. 71, the primitive circle within or upon which all poles will fall. As the c axis is projected at the center of the primi- tive, and is normal to the base, ooi, c at the center will be the pole of oo i. Draw two diameters aa', bb' at 90 ; these will represent the a and b axes. The poles of all the faces belonging to the zone of which c is the axis will fall on the primi- tive circle, as their normals will lie in the plane of the paper, the last term of their indices will be o. 100 will be projected at the ex- tremities of the axis fi, oio at the extremities of b; the remaining member of this zone (no) will lie symmetrically on either side of the axis b. Draw the two diame- ters at 78 22', and the poles of (no) will lie at their extremities. The form (on) is a member of the zone of which the axis & is the zonal axis ; its poles will lie on the diameter of the primitive, perpendicular to &, 52 23' from the pole c. From a' lay off a'e = 52 23', draw ea and where it crosses be is the pole on ; on will be an equal distance on the other side of c. The poles of (102) will lie on the diameter aa' and are located by the same method as (01 1) . The two pyramids (in), (112) are members of the zones no ooi and Iio ooi , their poles will lie on these two diameters. The form (in) is also a member of the zone on 100 and oil 100, its poles will be situated at the intersection of the two zonal circles. Draw the zonal circle 100, on, loo; where this intersects the zone no, ooi, no as at in will be the pole of the unit pyramid. Poles of (112) are located by the angle ooi A ii2 = 45 ; when the angle between the base and any face is given, the zone being known, its poles are quickly found by the tangent rule, or by construction, problem V. The advantages of the stereographical projection are, that it shows at once the symmetry of the crystal, connects all faces be- longing to the same zone, and by simple construction the angle between any two faces may be measured. CHAPTER III ISOMETRIC SYSTEM THE 32 TYPES OF CRYSTALS CRYSTAL forms, from a consideration of their symmetry alone, are grouped into 32 types ; when referred to their axes, they fall into six systems. The types which are included in any one system are independent, and one type cannot combine with another to form crystals even though they belong to the same system ; each type possesses a symmetry entirely its own, a characteristic derived from the molecular arrangement and molecule itself. At the same time all those included in one system are related by the possession of axes or planes of symmetry to a large extent common to the group. For this reason the old classification of holohedrons, hemihedrons, etc., is also given, and their derivation one from the other, as the best method for the beginner to obtain a clear understanding of the relation of types and the influence of planes and axes of symmetry in the development of crystal forms. Under the isometric system are grouped all those crystal forms which are referred to three equal interchangeable axes, intersecting at 90. It includes five types, all of which are characterized by at least four trigonal and three digonal axes of symmetry. Since the crystallographical axes are all equal, they are designated by the symbol a, and a becomes the unit of measurement on these axes. The most general set of parameters is therefore na : a : ma, in which the two variables n and m may have any value between unity, when the plane intersects that axis at unit length or a, and infinity, when the plane is parallel to that axis, or intersects it at infinity. From this standpoint all forms in the system may be considered as special cases of the most general form. CLASS, ISOMETRIC HOLOHEDRAL, HOLOSYMMETRIC, OR NORMAL TYPE 32, DITESSERAL CENTRAL This type possesses the highest symmetry possible in any crys- tal form, and, as in all holohedral classes, the forms are bounded by 47 48 MINERALOGY pairs of parallel faces. Diagram Fig. 72 represents the symmetry ; 13 axes, 9 planes, and a center. The value and position of the axes and planes may be understood best by the consideration of their relation to the edges of the cube or hexahedron. There are three ditetragonal axes ending in the center of the cube faces, these are the crystallographi- cal axes; four ditrigonal axes end- ing in the corners; six didigonal axes ending in the middle of the edges; three planes of symmetry bisect the edges of the cube, and contain the crystallographical axes ; they intersect in the center of sym- metry and divide space arou d it into eight equal portions (oc- tants). The remaining six planes contain the edges, each plane passing through the center and opposite edges. The nine planes of symmetry divide space around the center into 48 equal tri- angular solid angles. FIG. 72. Forms I. Hexoctahedron ; nararma; (hkl), Fig. 73. Here the values of the coefficients, m and n, are independent of each other and not at their limiting values, i or oo. When n = 2 and m = 3, yielding the parameters 3 a : a : 2 a, they will locate a face in each one of the 48 triangular segments into which the planes of symmetry di- vide space, or 48 sc.alene triangles. This is the largest number of faces possible on any crystal form. Eight faces are symmetrically grouped around the extremities of the ditetragonal axes (crystallographical axes) ; six around the ditrigonal axes (center of the octants) ; four around the di- FIG. 73. The Hexahedron, 3a:a:2a. digonal axes. All faces are similar scalene triangles, each of which intersects one axis at unity, the second at a greater dis- tance, the third at a still greater distance. Faces, edges, and ISOMETRIC SYSTEM 49 angles of the form will vary with the value of m and n, there is therefore a series of hexoctahedra, of which 3 a : a : 2 a is one. The spherical projection of the hexoctahedron is represented in Fig. 73 a. The planes of symmetry are represented by the great circles in which they cut the sphere of projec- tion. The poles of the faces in the northern hemisphere are represented by small circles, those in the south- ern hemisphere by crosses. The cross within the circle indicates that the two hemispheres are mirror images of each other, and the type is equatorial. The points at which the axes of symmetry emerge are indicated by the conventional signs. II. Tetrahexahedron ; na:a:ooa; (hko), Fig. 74. This form is a special case of (hkl), where 1 = o ; or each face cuts one axis at oo, one at unity, and one at an intermediate distance. If the poles of (hkl) are moved, so as to lie in the diametral planes, Fig. 73 a, two normals will coincide, as a with a' e FIG. 73 a. Hexoctahedron. FIG. 74. The Tetrahexahe- dron (320), of Fluorite. FIG. 75. Tetrahexahedron (320). or b with b', producing a form bounded by 24 isosceles triangles. Four faces are grouped around the ditetragonal axes, Fig. 75, and six around the di trigonal, and the didigonal axes bisect the basal edges between adjacent faces. The solid angles will vary with the value of n, yielding a series of tetrahexahedra, members of which may occur in combination. The form may also be considered as derived from the cube by replacing each face with four triangles. 50 MINERALOGY III. Tetragonal Trisoctahedron ; na:a:na; (hhl), Fig. 76. If in place of moving the pole of the most general form (hkl) to the diametral plane, it be now moved into the planes of symmetry which bisect the octants, and between the ditetragonal and ditrig- onal axes ; as the poles approach the plane the angle between the FIG. 76. The Tetragonal Trisocta- FIG. 77. The Tetragonal Tris- hedron, (hhl). octahedron. normals constantly diminish until the plane is reached, when it becomes 0, Fig. 73 a, and the angle between the faces they represent is 180. Thus two faces, b and c, a and e, of the most general form will coalesce, producing a form bounded by 24 four-sided faces, Fig. 76, having three faces entirely within each octant. Four faces are grouped around the ditetragonal, three around the ditrig- onal, and four around the didigonal axes. In this form, also, the solid angles between faces will vary with the value of n, yielding a series. The tetragonal trisoctahedron may be produced by re- placing each face of the octahedron with three tetragonal faces. IV. Trigonal Trisoctahedron ; a:a:na; (hhi), Fig. 78. Let the poles of the most general form now be moved till they lie in the plane of symmetry between the ditrigonal and didigonal axes, Fig. 73 a. Again two faces, a and b, a' and b', of the most general form will fall in one plane, producing still a third form bounded by 24 faces, Fig. 78; each face is an isosceles triangle with its base lying in the diametral plane. Eight of its faces, Fig. 79, are grouped around the ditetragonal, three around the ditrigonal, and the didigonal axes bisect the base of the triangular face. As in the preceding forms the solid angles vary with the value of n, producing a series of trigonal trisoctahedra. ISOMETRIC SYSTEM 51 V. Hexahedron; a : oo a : a; (ooi), Fig. 80. In the previous cases the poles of the most general form have been moved into one of the sides of the triangle, in which it lies ; there are still three possibilities, the three corners of the triangle. Let FIG. 78. The Trigonal Trisocta- hedron, (hhi). FIG. 79. Trigonal Trisocta- hedron, (hhi). it now be moved, Fig. 73 a, to coincide with the ditetragonal axis, when all eight faces of the most general form grouped around this, as b, b', c, etc., will fall in one plane, producing a form with six faces, the hexahedron, or cube, Fig. 80. Each face will cut one axis and is parallel to the other two. The ditetragonal axes will FIG. 80. The Hexahedron, (100). FIG. 81. The Rhombic Dodeca- hedron, (110). end in the center of the faces, the ditrigonal in the corners, and the didigonal will bisect the edges. The angles between the faces are fixed at 90, there is but one hexahedron and not a series. It is therefore termed a fixed form. 52 MINERALOGY VI. Rhombic Dodecahedron ; a:a:ooa; (no), Fig. 81. If the pole is now moved to the didigonal axes, Fig. 73 a, four faces a, a', b, b', will fall in one plane, producing a form with 12 rhombic faces, Fig. 81. The faces are grouped four around the ditetragonal axes, three around the ditrigonal, and the didigonal axes bisect the edges. There is but one rhom- bic dodecahedron with the angles fixed at 120. It is also a fixed form. VII. Octahedron; a:a: a; (in), Fig. 82. The seventh and last possible form in this type is where the pole is moved to the ditrigonal axes, when the six faces a, b, c, e, etc., of the general form grouped around this axis will fall in one plane, pro- ducing a form bounded by eight equilateral triangular faces, Fig. 82. Four faces are grouped around the ditetragonal axes, the ditrigonal axes terminate in the center of the face, and the didigonal axes bisect the edges. All dihedral angles of the regular octahedron are fixed at 70 31 ' 42" ; it is therefore a fixed form. FIG. 82. The Octahedron, (111). Relation of the Seven Forms When any one of the 48 triangular segments into which the planes of symmetry divide space is considered, Fig. 83, it has been shown that the pole na : a : ma, the hexoc- a: a: a tahedron, may be located anywhere within the area, and when it ap- proaches the sides or angles, either one or both of the variables m and n approach their limiting values 1 and oo. If the pole approaches one of the sides, only one of the variables approached its limit, or the two coa:aa variables are of the same value. The FlG - 83 - hexoctahedron, tetrahexahedron, tetragonal trisoctahedron, and the trigonal trisoctahedron, are known as the variable forms, since their parameters contain a variable. The position of the pole of ISOMETRIC SYSTEM 53 any one form on the triangle will depend upon the value of the variable. As the pole of any of the variable forms approaches the angle of the triangle, both variables approach their limits; and upon reaching their limits the pole assumes the position of one of the axes of symmetry, and as there is only one point in the angle of the triangle, therefore there is only one possible form of the hexahedron, rhombic dodecahedron, or octahedron, and they are known as the fixed, or limiting forms. All substances crystalliz- ing in these forms must have the same angle. In each type there are always seven possible forms. The most general form is represented by the area of the triangle ; and as the number of points which the pole of the face may occupy is unlimited, there are therefore innumerable individuals forming a series. The three sides of the triangle each represents a series of variable forms, as here also there is a large number of points on each side between the angles, each of which may be occupied by the pole in turn. The three angles of the triangle represent the three fixed forms, as there is only one point in each of the three angles. The seven forms possible in each type are represented by the seven elements of the triangle, of which the three angles represent the three fixed forms, and the three sides and area represent the variable forms. Combination of Forms Crystals may present one form only, when the number of faces is very limited ; more often they are combinations of two, three, or even all seven of the possible forms in the type, and in addition forms of the same series ; in such cases the number of faces possible *. -.kl FIG. 84. FIG. 85. on a crystal is very large and their relation complex. Such com- plex crystals are rare in nature, for by far the larger number are 54 MINERALOGY combinations of a few simple forms. The general appearance or habit is fixed by the simple form predominating in the combination. Figure 84 is a combination of a hexahedron and octahedron, the former predominating ; Fig. 85 is the same combination with the latter predominating. Examples crystallizing in the Type Copper, forms (100) (110) (111) (410) (211) (531). Fig. 86. FIG. 86. Combination of a (001) and d (110) on copper. FIG. 87. Magnetite, a (GUI) and d (110). Lead, forms (100) (110) (111) (410) (550). Silver, forms (110) (111) (310) (751). Galena, PbS, forms (100) (110) (111) (221). Fig. 84. Magnetite, Fe 3 O 4 , forms (100) (110) (111) (210) (221) (432). Fig. 87. Fluorite, CaF 2 , forms (100) (111) (421) (110) (211). Analcite, NaAl 3 (SiO 3 ) 2 , H 2 O, forms (100) (211) (575). n FIG. 88. Garnet ; Combi- nation of n (221) andd(HO). Fi g- CLASS, TETRAHEDRAL (DIAGONAL-FACED) HEMIHEDRONS TYPE 31, DITESSERAL POLAR All forms of this type possess four ditrigonal polar axes, terminat- ing in the center of the octants. The conditions surrounding one extremity of the axis are different from those at the other extrem- ISOMETRIC SYSTEM 55 ity, hence the term polar. The crystallographic axes are didigonal axes. The six planes of sym- metry bisect the octants and pass through opposite edges of the cube, Fig. 89. Symmetry, 4 ditrigonal polar axes, 3 didigonal axes, and 6 planes. There being no center of symmetry, the forms are not bounded by parallel faces. FIG. 89. Diagram of Axes and Planes of Symmetry in Type 31. Forms I. Hextetrahedron ; na : a : ma K (hkl) K (hkl). In grouping the faces around the isometric axes so as to conform to the symmetry of any type, it is necessary to cut all extremities of the crystallographical axes with the same number of faces and at the same inclination, since the axes are interchangeable. If planes are grouped on the axes, fulfilling the symmetry of this type, the most general form will be bounded by 24 similar scalene triangles, Fig. 90; six faces are grouped around the ditrigonal, FIG. 90. The Hextetrahedron, K(hkl). FIG. 91. The Hextetrahedron, K(hkl). four around the didigonal axes. The spherical projection, Fig. 91, shows that the poles (circles) in the northern hemisphere do not reflect those in the southern hemisphere (crosses) ; therefore the plane of projection is not a plane of symmetry. If this projection is compared with Fig. 73 a, it will be seen that the poles of the hextet- 56 MINERALOGY rahedron correspond to one half the poles of hexoctahedron. It is as if all the faces in alternate octants above and below were extended (Fig. 35, the shaded octants) until they inclosed space ; the form produced would be the + hextetrahedron. When the unshaded faces are extended the hextetrahedron is produced, congruent with the former by a revolution of 90. In all hemihedrons there are + and , or right and left forms, which may occur on crystals in combination, or independently. forms are always congruent by a revolution. Other forms of this type may be produced, as in the holohedral class, by moving the pole of the most general form to the sides and angles of the triangle in which it lies, yielding in all seven possible forms, some of which will be new forms ; others will be of the same shape as the holohedral forms. II. In Fig. 91, if all the poles be moved till they lie on the side of the triangles between the two clidigonal axes, they will occupy the same position as, Fig. 75, the poles bf .the tetrahexahedron. The holohedral and hemihedral forms are of the same shape, but the symmetry of the two, caused by the character or arrangement of the molecules, will differ. Where an apparent holohedral form is found in combination with hemihedral forms, it must be considered as a hemihedron and will possess the lower type of symmetry. The tetrahexahedron is reproduced by extending the faces which exist in alternate octants, as each face of the holohedral form extends in two octants, half in each ; the half lying in the octants extended will reproduce the half in the adjacent octants. There are no + or forms in those cases where the hemihedron assumes the holohedral shape. III. Trigonal tristetrahedron ; na a na . If the poles be placed on the side of the triangle between the ditrigonal and didigonal axes, two will coincide, yielding a new form, the trigonal tristetrahedron, bounded by 12 simi- lar isosceles triangles, Fig. 92. Three faces are grouped around one ex- tremity of the ditrigonal axis and 6 around the other. The didigonal FIG. 92. The Plus Trigraal Tris- axes bisec t the base of the triangu- tetrahedron. lar faces. This form may also be ISOMETRIC SYSTEM 57 considered as produced by the extension of alternate octants of the tetragonal trisoctahedron. There are congruent + and forms. IV. Tetragonal tristetrahedron ; na : a : a K (hhi) K (hhi). Let the poles now be moved on the side of the triangle between the ditrigonal axes, when the tetragonal tristetrahedron bounded by 12 tetragonal faces will be pro- duced, Fig. 93. Three faces are grouped around the extremities of the ditrig- onal, four around the didigonal axes. This form may also be derived by ex- tending alternate octants of the trigonal trisoctahedron. The four tetrahedral forms thus far considered are variable forms, as their angles will depend upon the value of the intercepts. V. If the pole be placed on the didigonal axes, the hexahedron will be reproduced, as in type 32. VI. If the pole be placed in the plane of symmetry midway between the ditrigonal axes, the rhombic dodecahedron will be re- produced ; both hexahedra and rhombic dodecahedra may combine with tetrahedral forms. A A * O VII. Tetrahedron; - - ; K (in) K (111). FIG. 93. Tetragonal Tris- tetrahedron, K (221). FIG. 94. The Plus Tetrahedron, K(lll). FIG. 94 a. The Negative Tetra- hedron, K(lll). If the poles be placed on the ditrigonal axes, six faces will fall in the same plane, producing a form bounded by four equal equi- lateral triangles, the regular tetrahedron, Fig. 94. It may also be 58 MINERALOGY considered as produced by the extension of alternate faces of the octahedron. Combinations Of the seven forms which may combine in the tetrahedral class, the tetrahexahedron, hexahedron, and rhombic dodecahedron are apparently holohedrons in shape. They can be distinguished from the latter when not in combination with tetrahedral forms only by special markings, striations, etch figures, or other physical proper- ties, which indicate a symmetry of the lower type. Crystal faces often contain striations, parallel to the edge of common combinations found represented on the crystals. They FIG. 95. FIG. 96. The Plus and Minus Tetrahedrons of Sphalerite. are attributed to alternations of growth, in which the face is reduced to the minimum, and appear as a striation parallel to the common edge. Fig. 95 represents a combination of the hexahedron and tetrahedron in sphalerite. The striations on the hexahedral faces <2 FIG. 97. Combination of K(lll), K(211), (110) on Tetrahedrite. FIG. 98. Combination of (100), K(lll), K(211) on Boracite. are traces of tetrahedral faces. It is to be noted that the striations on the cube faces are symmetrical to planes containing opposite ISOMETRIC SYSTEM 59 edges of the cube, which mark it as a hemihedral cube belonging to the tetrahedral type of symmetry, even if the tetrahedron did not truncate the corners. Examples of minerals crystallizing in the type. Diamond, C, (ill) (100) (321) (210) (320). Sphalerite, ZnS, (100) (111) (110) (311) (331) (210). Fig. 96. Tetrahedrite, 4Cu 2 S, SbsSa, (100) (110) (111) (221). Fig. 97. Boracite, (100) (110) (111) (410) (531) (221). Fig. 98. CLASS, PYRITOHEDRAL (PARALLEL-FACED) HEMIHEDRONS TYPE 30, TESSERAL CENTRAL All forms of this type possess four trigonal axes ending in the center of the octants; three di- digonal axes corresponding to the crystallographical axes; three planes, the diametral planes and a center of symmetry. All forms of the type are therefore bounded by pairs of parallel faces. Symmetry. 4 trigonal axes, 3 didigonal axes, 3 planes and a center, Fig. 99. Forms FIG. 99. I. Diploid; na : a : ma IT (hkl) TT (hkl.) If planes be grouped on the axes to conform to the symmetry of the type, it will be found that if half of the faces of the hexocta- FIG. 100. The Minus Dip- loid, IT (hkl). FIG. 100 a. 60 MINERALOGY hedron are selected, so that pairs taken intersecting in the diame- tral planes (Fig. 37, the shaded faces) are then extended, they will produce a new form, the diploid, Fig. 100, with 24 four-sided faces, three of which are grouped around the trigonal and four around the didigonal axes. Figure 100 a shows the symmetry and poles of the form. II. Pyritohedron ; pentagonal dodecahedron ; ir(hlo) ir(hlo). When the pole is moved to the side of the triangle between the didigonal axes, and in the plane of symmetry, a new form will be produced, the pyritohedron, Fig. 101, bounded by 12 pentagonal faces. Three faces are grouped around the trigonal axes, and the di- digonal axes bisect the long edge between adjacent faces. The FIG. 101. The Pyritohedron, ir(hlo). FIG. 102. Pyrite: Combi- nation of (100) and IT (hlo). pyritohedron may be considered as derived from the tetrahexahe- dron by extending alternate faces. III. Other forms. Other possible positions of the poles are identical in number and positions with the forms of type 32: Therefore the tetragonal trisoctahedron, trigonal trisoctahedron, hexahedron, rhombic dodecahedron, and octahedron may be found in combination with the diploid and pyritohedron. They also reproduce the same forms when the method of selection to form hemihedrons of this class is applied to them. Combinations Geometrical holohedral forms of this type must possess the pyri- tohedral symmetry. Pyrite, FeSa, crystallizes in all seven forms of ISOMETRIC SYSTEM 61 the type, but commonly in (111) (010) TT (hlO) with striations on the cube face, parallel to its edges, due to alternations of growth be- tween the cube and pyritohedron, Fig. 102. These striations are parallel to the planes of symmetry which bisect the cube edges, and not, as in sphalerite, type 31, parallel to the planes which con- tain the edges and cross the face diagonally. Other representatives of the type are smaltite, CoAs2 ; cobaltite, CoAsS. CLASS, PLAGIOHEDRAL (GYROIDAL) HEMIHEDRONS TYPE 29, TESSERAL HOLOAXIAL As the name implies, this type possesses all the axes, 3 tetragonal, 4 trigonal, 6 digonal, of the system, but no planes or center of symmetry. Forms I. Pentagonal icositetrahedron (didodecahedron) r/1 - T (hkl) T (khl). If every other face of the hexoctahedron around the ditetragonal axis is extended, as indicated by the shaded faces of Fig. 39, the solid formed will have the symmetry of this type. It is bounded by 24 pentagonal faces, 4 of which are grouped around the tetrag- onal axes, 3 around the trigonal, and the digonal axes of symme- try bisect the edge between two faces. When the right upper face of the positive octant is extended, then the right pentagonal dido- decahedron is produced, Fig. 103. If the left upper face is ex- FIG. 103. The Right Pen- FIG. 104. The Left Pen- tagonal Didodecahedron. tagonal Didodecahedron. tended, the left pentagonal didodecahedron, Fig. 104, is produced. Figure 105 is a spherical projection of the right form. They differ from + and hemihedra, as there is no way of revolving one into 62 MINERALOGY a congruent position with the other ; they are not superimpos- able; one is a mirror image of the other. Such pairs of forms are enantiomorphous. ....^ Other forms of the type do not .'' / \ \ ' differ geometrically from those of \. ''. tyP e 32, the forms which may be ..-4 X T" "":4--.. X \ found in combination with the pen- */o\ : -' * : "x '; tagonal didodecahedron, the only new ^:. x , : -. o t x form of the type will be the tetra- ''-.. \/o I x *'!.- / hexahedron (hlo). \ *,;-*. "*"To / '\ / Tetragonal trisoctahedron (hhl). \ '/ Trigonal trisoctahedron, (hhi). --...^....-""'' Hexahedron, (010). FIG. 105. The Right Pentagonal Rhombic dodecahedron, (110). Icositetrahedron. Octahedron, (111). As six forms of the type are in shape holohedrons, minerals which crystallize in the type are distinguished by a study of the symmetry of the etch figures; this is especially necessary as the occurrence of the most general form is always rare. Sylvite, KC1, occurs in combinations of the cube and octahedron, Fig. 106. Etch figures appear on the cube faces as shallow pits with square outline. The position of ^v the square pits depends upon the sym- metry of the point-system, as the type FIG. 106. Diagram of Etch contains no planes of symmetry; they Figures on Sylvite. are so oriented on the cube that none of the possible planes which are traced on the cube face will cut them symmetrically. Examples. Cuprite, Cu 2 ; (111) (100) (110) (211) rarely in T (hkl), from Cornwall, Eng., Ammonium Chloride NH^Cl; (111) (110) (100) r(875). TETARTOHEDRAL CLASS TYPE 28, TESSERAL POLAR Symmetry : crystals of this type must conform to 4 trigonal axes ending in the center of the octants; these four trigonal axes maintain their position in all five types of the isometric system and to three digonal axes corresponding to the crystallographic axes. They have no planes or center of symmetry. ISOMETRIC SYSTEM 63 na : a : ma Forms Tetartohedral pentagonal dodecahedron ; R, L KIT (hkl), KIT (hkl), KIT (khl), KIT (khl). The symmetry of the type requires but one quarter of the faces of the holosymmetric form. Figure 107 represents the poles of the + right tetartohedral pentagonal dodecahedron. The form is bounded by 12 irregular pentagonal faces, three of which are grouped around the trigonal axes; the digonal axes bisect an edge. 4L -B +R -L -L +R -B : -f-L FIG. 107. The Plus Right Tet- rahedral Pentagonal Dodeca- hedron. FIG. 108. Figure 108 represents the 8 faces of the hexoctahedron grouped around the ditetragonal axis, which axis in the tetartohedral class is a digonal axis. Four tetartohedral pentagonal dodecahedra are possible. If the two faces + R are extended, the + right, FIG. 109. The Plus Right Tetartohedral Pentagonal Dodecahedron, KIT (hkl). FIG. 110. The Plus Left Te- tartohedral Pentagonal Do- decahedron, irK(khl). Fig. 109, pentagonal dodecahedron results ; if + L are extended, the + left, Fig. 110 ; -- R is the negative right, - L the negative left forms. rights are congruent, for if the face + R is revolved 64 MINERALOGY 90 it is superimposed on R, likewise lefts are congruent, but the rights are enantiomorphic with the lefts, for in no way can the face R be revolved around the digonal axis to bring it into a congruent position with the face + L or L. There are always four tetartohedral forms. They may be derived by superimposing one hemihedral type of selection on another and extending the faces remaining, Fig. 41, page 27. Other Forms The six other possible forms of the type are derived by a consid- eration of the position of the poles in the triangle, Fig. 107 : Pole on the side between the digonal axes = + pyritohedron. Pole on the side between the digonal and trigbnal axes = tetragonal tristetrahedron. Pole on the side between trigonal axes = trigonal tristetrahe- dron. Pole on the digonal axes = hexahedron. Pole on the trigonal axes = tetrahedron. Pole on the angle between the digonal axes = rhombic dodeca- hedron. Combinations Apparent holohedral and hemihedral forms of more than one type may be found combined on the same crystal ; when this is observed, all forms must be considered as being of tetartohedral symmetry. Examples : Minerals crystallizing in the type are rare. Ullmannite from one locality crystallizes in pyritohedra and in tetrahedra from another, which would indicate tha^ it is tetarto- hedral. There are a number of artificial salts, as barium nitrate, sodium chlorate, strontium nitrate, and sodium bromate, which crystallize in this type. CHAPTER IV TETRAGONAL SYSTEM THE tetragonal system embraces all those crystals referable to three axes at right angles, two of which, the lateral axes, are equal and interchangeable, designated by the letter a. The third or vertical axis, designated by c, is not interchangeable with the lateral axes. The most general parameter of the system is na : a : me, in which the value of n may vary from unity to infinity; m may vary from zero to infinity. With the c axis held vertical and one of the lateral axes in the plane of the paper, the other at 90 to it, the extremities are designated + or as in the isometric sys- tem, the upper right octant being positive. Seven of the 32 types are included in the tetragonal system. CLASS, TETRAGONAL HOLOHEDRAL (HOLOSYMMETRIC) TYPE 27, DITETRAGONAL EQUATORIAL Symmetry. Crystals of this type possess one axis of ditetrag- onal symmetry, the c axis ; four didigonal axes, two of which are FIG. 111. The Planes of Symmetry in Type 27. FIG. 112. The Ditetragonal Pyramid. the lateral axes; the other two, the intermediate axes, bisect the angle between the a axes. All four didigonal axes lie in one plane, the equatorial plane, at 90 to the vertical axis. There are five P 65 66 MINERALOGY planes of symmetry, Fig. Ill, one of which is the equatorial plane; the other four intersect in the c axis and each contains one of the axes of symmetry lying in the equatorial plane. The five planes divide space into 16 equal (Fig. 112) triangular portions, eight above and eight below the equator. The largest number of faces possible upon any form of the tetragonal system will be 16. Forms I. Ditetragonal pyramid ; na : a : me ; (hkl). When the values of n and m are between their limits, the pole of the face will fall within the area of the triangle, Fig. 112 ; there will be one face in each triangular space, yield- ing a form, the ditetragonal pyramid, Fig. 113, bounded by 16 scalene triangles (pyramid here includes the faces above and below the equator and are doubly pointed). It has eight faces grouped around the north and eight around the south pole or c axis ; four faces grouped around the extremities of the didigonal axes. There is a series of ditetragonal pyramids, the shape of the face or the value of the interfacial angles of any one of which will depend upon the values of n and m. II. Tetragonal pyramid of the first order; a : a : me ; (hhl) . When the value of n is unity, its minimum FIG. 113. The Ditet- ragonal Pyramid. limit, or if the pole in the spherical projection, Fig. 112, is moved until it coincides with the intermediate axes, then two adjacent poles of the most gen- eral form, as a and c, will combine, yielding a form bounded by eight isosceles triangles, Fig. 114, the tetragonal pyr- amid of the first order. Its eight polar edges are equal. The crystal- lographical axes termi- nate in a solid tetrahe- dral angle; this char- FIG. lU.-Pyramid of the First Order (111), of acterizes a pyramid of Cassiterite. the first order; in pyramids of the second order the a axes bisect TETRAGONAL SYSTEM 67 FIG. 115. Pyramid of the Second Order, (101), of Cassiterite. an edge. There is a series of pyramids of the first order, their acuteness and general appearance depending upon the value of m. III. Tetragonal pyramid of the second order ; a : oo a : me ; (hoi). When the value of n is , its maximum limit, or if the pole is moved to coincide with the crystallographical axes, then in the resulting form of eight faces the axes will ter- minate in the center of the equatorial edge, yielding a pyramid of the second order, Fig. 115. In shape this pyr- amid in no way differs from the pyramid of the first order, with which it becomes congruent by a revolution of 45 around the c axis. There is a series of pyramids of the second order, depending upon the value of m. IV. Ditetragonal prism; na:a:c; (hko). When the value of n is between its limits and m is infinity, or if the pole in the spherical projection is moved to the primitive circle between the extremities of the didigonal axes, the resulting form is the di tetragonal prism, Fig. 116. It is bounded by eight similar faces. Each face will cut one of the lateral axes at unity, the other at a distance greater than unity, and will be par- allel to the c axis; it will therefore be an open form extending to infinity unless ter- minated by combining with another form. All prisms are open forms. There is a series of ditetragonal prisms, the value of the interfacial angles depending upon the value of n. V. Tetragonal prism of the first FlG . 117 ._ Prismo f the First order; a:a:ooc; (no). Order, (HO). V-X ; ! ^^ L ^\ ~"f FIG. 116. The Di- tetragonal Prism, (210). 68 MINERALOGY FIG. us. The Tetragonal Prism of the Second Order. When the value of n is unity and that of m is infinity, or let the pole be moved on. the equatorial plane to coincide with the inter- mediate axes, then the resulting form is the tetragonal prism ^ of the first order, Fig. 117. It will be bounded by four faces, cutting the c axis at infinity, the a axes at unity. The lateral axes terminate in the middle of the edges. VI. Tetragonal prism of the second order; ooa:a:ooc; (oio). When the value of both n and m is at infinity, their maximum limit, or if the pole be moved in the equatorial plane to coin- cide with the crystallographic axes, then a rectangular prism results, the tetragonal prism of the second order, which in shape differs in no way from the prism of the first order except the a axes terminate in the center of the faces. It becomes congruent with the first order prism by a revolution of 45 around the c axis, Fig. 118. VII. Basal pinacoid; ooa: ooa:c; (ooi). The only possible position of the pole remaining is when it coin- cides with the c axis, when all eight faces above the equato- rial 'plane will form one face and all the faces below will fall in one plane, yielding a form of two faces, the tetragonal base or basal pinacoid which extends to infinity on all sides, Fig. 118, c. All pinacoids cut but one axis and are parallel to the other two ; in combination with prisms they inclose space. The fixed forms of the tetrago- nal system are the base and the prisms of the first and second orders ; these correspond to the fixed forms of the isometric system, as do also the position of their poles in the triangle, viz., the corners or angles; as here there is evidently only one position for the pole, there is only one form possible. FIG. 119. Combination of (111) (110) (100) of Cassiterite. (101) TETRAGONAL SYSTEM 69 Combinations. The pyramids and prisms of the first 'and second orders truncate each other's edges symmetrically; Fig. 119 repre- sents these four forms of cassiterite. Several members of a series of a variable form may occur in combination with the fixed forms, Fig. 120, Zircon. Examples of minerals crystallizing in the di- tetragonal equatorial: Cassiterite, Sn0 2 ; (110) (100) (310) (111) (101). Zircon, ZrSi0 4 ; (110) (100) (111) (331) (311). Rutile, TiO 2 ; (110) (100) (310) (111) (101). Vesuvianite, Ca(Al, OH) Al 2 (Si0 4 ) 5 ; (100) (110) (310) (210) (001). FIG. 120. Combina- tion of (111) (331) (311), (110) and (010) in Zircon. CLASS, SPHENOIDAL (DIAGONAL-FACED) HEMIHEDRONS TYPE 26, DITETRAGONAL ALTERNATING Symmetry. Crystals of this type possess a ditetragonal alter- nating axis, the c axis; two digonal axes, the a axes; and two planes of symmetry intersecting in the c axis and each containing one of the intermediate lateral axes; Fig. 121 represents the symmetry of the type. this class of hemihedrons in the tetragonal system corre- sponds to the tetrahedral class FIG. 121. Type 26: Ditetragonal ., , . alternating. m the ISOmetriC system and may be considered as derived from the holohedral FIG. 122. The Minus f , ,. ., |, Scalenohedronic(hkl). forms by extending all the faces in alternate octants, Fig. 122; the shaded faces when extended will produce a form as drawn. 70 MINERALOGY I. Tetragonal scalenohedron ; na : a : me K(hkl)K(hkl). FIG. 123. The Plus Sphenoid ic(121). This form is bounded by eight similar scalene triangles, Fig. 123, four of which are grouped around each extremity of the c axis, with the edges lying in the planes of sym- metry. The lateral axes terminate in the middle of an edge. There are + and forms which become congruent by a revolution of 90 around the c axis. II. Sphenoid; a : a: me; K(hhl)K(hhl). When the pole is in the plane of symmetry repre- sented by the full lines, Fig. 121, two faces of the scalenohedron will fall in one plane, as e and e' and b, b', and the form will be bounded by four FlG 124 _ The Plus isosceles triangles, Fig. 124, producing the tet- Tetragonal Sphenoid of ragonal sphenoid of the first order in which the the First Order, K(IH). lateral axes bisect the four equal edges. The c axis bisects the remaining two edges. This form may also be considered as derived from the tetragonal pyramid of the first order by extending alternate faces above and below until they inclose space. III. Other forms of the type. All other positions of the pole will in turn produce FIG. 125. Combination the holohedral shapes of (122) and (772) of of t yp e 27, here as in Chalcopyrite. , , . the -isometric system; here, also, they must be considered as of lower symmetry, which will be recognized by their combination with the two new forms of the type, or by their striations and Fl ^ 126 ' ~~ Chalc py rite : etch figures. Combination of (111) TETRAGONAL SYSTEM 71 The possible forms to be found in combination in this type are : The positive and negative tetragonal scalenohedrons, K (hkl), K(hkl). The positive and negative sphenoids of the first order, K(hhl), K (hffl). The pyramid of the second order, (ohl). ^' The ditetragonal prism, (hko). The prism of the first order, (hho). The prism of the second order, (oho). The basal pinacoid, (ooi). Examples of minerals crystallizing in the type. Chalcopyrite, FeCuS 2 ; (111) (111) (122) (772). Figs. 125 and 126. CLASS, HOLOHEDRAL HEMIMORPHIC TYPE 25, DITETRAGONAL POLAR Hemimorphic forms may be considered as derived from an equa- torial form by the suppression of all the faces around one extremity of the c axis and the development of those around the opposite extremity, as an independent form, or if the equatorial form is cut along the equatorial plane, it will yield an upper hemiform and lower hemiform. Symmetry. All the elements of symmetry in the equatorial plane are lost, namely the four axes, the equa- torial plane, and the center. The type is symmetrical in regard to a ditetrag- onal axis, the c axis and four planes of symmetry intersecting in the c axis, Fig. 127. Forms Each pyramid of the ditetragonal equatorial type yields an upper and lower hemipyramid, each of which is independent. The basal pinacoid is also divided into upper and lower forms. The possible forms to combination in this type are therefore : The upper and lower ditetragonal pyramid; u/1 - ; (hkl) (hid). FIG. 127. Type 25: Ditetrag- onal Polar. 72 MINERALOGY The upper and lower pyramid of the first order ; u/1 (hhl) (hhf). The upper and lower pyramid of the second order ; u/1 (ohl) (ohl). a i a ! me GO a : a : me . FIG. 128. The Upper Di- tetragonal Hemipyramid. FIG. 129. The Upper Hemi- pyramid of the First Order. The ditetragonal prism ; na : a : oo c ; (hko). The prism of the first order ; a : a : me ; (110). The prism of the second order ; oo a : a : oo c ; (010). The upper and lower basal pinacoid; u/l*" :ao>:c ; (001) Examples. There are as yet no minerals known to crystallize in this type, but several organic and artificial salts belong here. Fig. FIG. 130. Silver Fluor- ^39 represents a crystal of silver fluoride, in com - (113). bination. CLASS, PYRAMIDAL (PARALLEL-FACED) HEMIHEDRONS TYPE 24, TETRAGONAL EQUATORIAL Symmetry. Crystals of this type have an axis of tetragonal symmetry, the c axis ; one plane, the equator, and a center, Fig. 131. The forms may be considered as derived from the ditetragonal equatorial type by extending alternate pairs of faces which inter- sect in the equator, as is shown in Fig. 132, where the shaded faces when extended produce the third order pyramid, Fig. 133. TETRAGONAL SYSTEM 73 Forms I. Tetragonal pyramid of the third order ; na : a : me (hkl) ir(hkl). The faces represented by the poles in Fig. 131 form a pyramid similar in shape to the tetragonal pyramids of the first and second orders, differing only in that the lateral axes terminate in the equatorial edge between the tet- rahedral angle and the middle. A diagram of the equatorial plane is represented in Fig. 134; aa is the pyramid of the first, ee is the pyramid of the second order, both being fixed forms ; ab is the ditetragonal pyramid, a variable form lying between the fixed forms as its limiting forms ; the dotted line represents the ex- tension of one half of the faces, as ab to d, where it intersects at FIG. 131. Type 24 : Tetragonal Equatorial. FIG. 132. FIG. 133. Pyramid of the Third Order of Scheelite, (313). FIG. 134. 90 with the extension of the alternate face ad ; note that the axes in this pyramid terminate not in the angle, nor in the middle of the edge, but at a point on the edge somewhere between them. This diagram will represent the relation of the prisms equally well. MINERALOGY II. Tetragonal prism of the third order; - '^ ; IT (hko) IT (hko). If the pole be placed on the primitive circle in an asymmetric position in the octant, or if every other face of the ditetragonal FIG. 135. Prism of the Third Order of Stolzite, (340). FIG. 135 a. Combination of the First Order Prism with the Third Order Pyramid. prism is extended, a new form will result, the tetragonal prism of the third order, Fig. 135, similar in shape to the 1st and 2d order prisms, except the lateral axes end in the face between the center of the face and the middle of the edges. All other positions of the pole will yield holohedral shapes. The possible forms in combination will therefore be the Positive and negative pyramids of the 3d order, IT (hkl) ir(hkl). Tetragonal pyramid of the 1st order, (hhl). Tetragonal pyramid of the 2d order, (hoi) . The positive and negative prism of the 3d order IT (hko), (hko). Tetragonal prism of the 1st order, (hho). Tetragonal prism of the 2d order, (oho). Basal pinacoid, (001). FIG. 136. Combination of (101) (111) and (313) of Scheelite. TETRAGONAL SYSTEM 75 Examples. In combinations the two new forms, the 3rd order pyramid and prism will give the crystals an asymmetric appearance as in Fig. 135 a. Examples. Scheelite; CaWO 4 : (101) (111) (313). Fig. 136. CLASS, TETRAGONAL TRAPEZOHEDRAL (PLAGIOHEDRAL) HEMI- HEDRONS TYPE 23, TETRAGONAL HOLOAXIAL Symmetry. As the name implies, this type has all the axes of symmetry of the tetra- ...-T- gonal system, but no 'y planes or center. The c axis is a tetragonal \ axis and the lateral I and intermediate axes o : are digonal axes of / symmetry. As in all ., / holoaxial types, there S are right and left enantiomorphic forms. : O FIG. 137. Type 23 : Tetragonal Holoaxial. Forms r/l na : a : me I. Tetragonal trapezohedron; T(hkl) T(khl). If alternate faces of the di- tetragonal pyramid around the north pole, represented by circles in Fig. 137 and shaded in Fig. 138, and faces alternating with these around the south pole, rep- resented by + in Fig. 137, are extended, the right trapezohedron, Fig. 139, will result; if the un- shaded faces of Fig. 138 are ex- tended, the left trapezohedron will result, Fig. 139 a. The form is bounded by eight 13 TetragKiai tra P ezoidal faces > four of which Trapezohedron. are grouped around each extrem- FIG. 138. FIG. FIG. 139 a. The Left Tetragonal Trapezohedron, r(212). 76 MINERALOGY ity of the c axis. The equator is represented by eight zigzag edges. The lateral axes bisect opposite edges. II. Other forms. All other positions of the poles will yield holohedral shapes. The trapezohedron is the only new form of the type, and it has never been found on a crystal. All substances crystallizing in this type have been placed* here as a result of a study of their etch figures. Forms possible to combine in the type are : The right and left tetragonal trapezohedron, r(hkl) r(khl). The tetragonal pyramid of the first order, (hhl). The tetragonal pyramid of the second order, (ohl). The ditetragonal prism, (hko). The tetragonal prism of the first order, (hho). The tetragonal prism of the second order, (oho). The tetragonal base, (001). Examples. There are no minerals of this type. The artificial nickel sulphate, NiSCX, 6 H 2 0, is placed here, also the sulphate of strychnine. CLASS, TETRAGONAL SPHENOIDAL (TETARTOHEDRAL) TYPE 22, TETRAGONAL ALTERNATING The c axis in this type is a tetragonal alternating axis ; there is no plane or center of symmetry. The forms of the type may be considered as derived from the holohe- dral forms by an extension of one quarter of the faces selected as illustrated in Fig. 140, which also shows the alternating character of the c axis. I. Sphenoid of the third order ; r/1 ^ ' a : mC ; TTK(hkl), TTK(khl), TTK(hkl), irK(khl). The faces represented by the poles of Fig. 141 produce the + R sphenoid of FIG. 140. The Plus Left Te- the third order, Fig. 142. The -f L form tragonai Sphenoid of the i s shown in Fig. 140, in its relation to the ditetragonal pyramid, where the four faces selected include the left-hand face of the + octant. There are four sphenoids of the third order possible, bearing the same relation to each other as in the tesseral polar type. The lateral TETRAGONAL SYSTEM 77 axes terminate asymmetrically in the face, on a line connecting the middle points of the four equal edges. I. Sphenoid of the second order ; - ; iTK(hol), TTK(ohl). When the poles of Fig. 141 are moved into the diametral planes, the + right and left sphenoids will fall in one form, the -f- sphenoid of the second order, Fig. 142, in which the lateral axes o : FIG. 141. Type 22: Tetragonal alternating. FIG. 142. The Te- tragonal Sphenoid of the Second Order. FIG. 142 a. The Plus Right Tetragonal Sphenoid of the Third Order. terminate in the central point of the line joining the middle of the four equal edges. Sphenoids of the second order may be considered as derived from the tetragonal pyramid of the second order by extending alternating faces above and below. II. Other forms. No new forms are produced by the other possible positions of the poles. Forms possible to^ combine in the type are : right and left sphenoids of the third order, irK(hkl), TTK(khl), TTK(hkl), "irK(khl). . sphenoid of the second order, K (ohl), K (hoi). sphenoid of the first order, K (hhl), K (hiil). prism of the third order, IT (hko), IT (kho). Prism of the second order, (oho) . Prism of the first order, (hho). Basal pinacoid, (001). Example. There is yet no representative of this type, either among minerals or artificial compounds. 78 MINERALOGY CLASS, HEMIHEDRAL HEMIMORPHIC TYPE 21, TETRAGONAL POLAR Symmetry. There is one axis of tetragonal symmetry ; all the symmetry of the equatorial plane is lost. This type is related to the tetragonal equatorial in the same way as the ditetragonal polar is to the di tetragonal equatorial, Fig. 143. Forms o \ I V ; i Tetragonal hemipyramid of the third order, u/1 na : a : me ir(hkl). FIG. 143. Type 21: Tetragonal Polar. The tetragonal pyramid of the third order yields the only new forms of the type, the positive, Fig. 144, and nega- tive upper and lower hemipyramids of the third order. Forms possible to combine in the type are : upper and lower hemipyramids, of the third order, ir (hkl) ir(khl), IT (hkl), ir(khl). Upper and lower hemipyramids of the second order, (ohl), (ohl). Upper and lower hemipyramids of the first order, (hhl), (hhl). prisms of the third order, ir(hko), ir(kho). Prism of the second order, (oho). Prism of the first order, (hho). Basal pinacoid, (001), (001). Examples. -rWulfenite, PbMo0 4 , (111) (111) (001) (430), Fig. 145. Etch figures. In the seven types of the tetragonal system it will be noted that two forms, the prisms of the first and second orders, are common to all. They are of the same shape and possess the same number of faces in each type ; and in external appearance the prism of one type is not to be distinguished from that of another. The same condition exists in the case of the cube and rhombic dodecahedron in the five FIG. 144. The Upper Hemipyra- mid of the Third Order, w(hkl). FIG. 145. Wulfenite (111) (111) (001) (430) TETRAGONAL SYSTEM 79 types of the isometric system. While outwardly these seven prisms of the first order are exactly alike, yet inwardly they all possess their distinctive symmetry. The physical properties are distributed on the face of each prism in accord with, and they conform to, the symmetry of the type. The seven prisms of the first order will differ then in their symmetry. When a solvent is applied to a crystal face or a natural crystal- line surface, as a cleavage surface, the crystal will not pass into solu- tion equally or with the same speed in all directions. T-he rapidity with which molecules pass into solution or are torn off from the crys- talline network will depend upon the symmetry of the network. The solvent's action will not act evenly all over the surface, but will start at points scattered over the crystal face ; and solution will begin at each one of these points as a nucleus or center, the molecules FIG. 146. Photograph of Etch Figures on Halite, enlarged Five Diameters. going into solution one after the other, with a speed that varies with the direction. If the solvent's action is stopped after a very short time and the crystalline surface is examined, in many cases, if the concentration and character of the solvent has been favorable and it has not been allowed to act too long, the surface will be pe- culiarly pitted. All of these pits or etch figures will be of the same shape on all faces of the same crystal form. They are bounded by straight or slightly curved lines and are of the nature of negative or reentrant crystals, with their equivalent faces and axes arranged parallel, as is shown in Fig. 146. These etch figures may be produced by plunging a crystal into a solvent for a very short time, as they are the result of the first action of the solvent on the crystal face. r MINERALOGY If solution is allowed to continue for a longer time, the outlines of the individual pits will meet and the face is then' often covered with characteristic hillocks, representing points where the walls of *.. [N the etch figures have not as yet joined. The sides of the etch fig- ures represent possible crystal faces, or more correctly vicinal faces, and they therefore reflect and conform to the symmetry of the face. Their shape will depend upon the chemical compound, the TETRAGONAL SYSTEM 81 strength and character of the solvent, as well as upon the crystal form etched ; but, however produced, if produced under like con- ditions, the etch figures on all faces of any one form will be alike and conform to the symmetry of the face. Etch figures are there- fore one of the most reliable means of determining the symmetry of any crystal face, and often decide the type to which a crystalline compound will belong when the crystal forms or combination of forms fail, and when the most general or distinctive form of the type is absent. Nickel sulphate, NiSO 4 , 6 H 2 O, has been placed in the tetragonal holoaxial type from the symmetry of its etch figures alone, while the most general or distinctive form of this type, the tetragonal trapezohedron, has never been observed on any crystal, and from the combination of forms alone it might belong to type 27, ditetragonal equatorial, or to type 24, the tetragonal equatorial. As an illustration of the method of determining the symmetry of apparently holohedral hemihedrons, Fig. 146 a represents the seven tetragonal prisms of the first order, with diagrammatic etch figures on each, conforming to the symmetry of the face in each of these seven possible prisms in the tetragonal system. The planes of sym- metry where they cross the prism face are represented by dotted lines and where they cross the etch figure by a white line. In 27, the ditetragonal type, the prism face is symmetrical to two planes of symmetry, the vertical and equatorial planes and a center of symmetry. The shape of the etch figures on this face a, as indi- cated, must be symmetrical to planes parallel to these two planes and an axis of symmetry ; furthermore, when the etch figure a is revolved 90 about the vertical axis c, it must become congruent with the etch figures, as a', on the adjacent prism face of the same form. If all these conditions are fulfilled, then the prism is of type 27. In type 26, the ditetragonal alternating, where the prism face is crossed by the vertical plane of symmetry only and the equatorial plane of symmetry is absent, the etch figures will be of a different shape, as represented, from those on the prism of type 27. They will be without an axis of symmetry, but will be symmetrical to the vertical plane ; and when revolved 90 around the vertical axis c and reflected over the equatorial plane, they will become congruent with the etch figures, as a, on the adjacent prism face. In this type an axis of digonal symmetry ends in the edge ; the etch figure a, if revolved around this axis 180, will be congruent with a'. They are oppositely oriented on adjacent faces. 82 MINERALOGY In type 25, the ditetragonal polar, where there is a vertical plane of symmetry crossing the prism face and the vertical axis is a dite- tragonal axis and no equatorial plane or digonal axes, the etch figure on each face is symmetrical to a vertical plane only ; and re- volved around the vertical axis c 90 will become congruent with those on the adjacent face, as a', a will also be a reflection of a', across the plane of symmetry containing the edge between them. In type 24, the tetragonal equatorial, where the vertical axis is a tetragonal axis of symmetry and has an equatorial plane, the etch figure will be symmetrical to the equatorial plane By a revolution of 90 around the vertical axis it will become congruent with a 7 , on the adjacent prism face, but a is not a reflection of a', as here there is no plane of symmetry between them. In type 23, the tetragonal holoaxial, where there are no planes of symmetry and the vertical axis is a tetragonal axis and there are four digonal axes in the equatorial plane, the etch figure must have a center of symmetry; and if revolved 90 around the vertical axis or 180 around the crystallographical axis, it must become con- gruent with a', the etch figures on the adjacent crystal faces. The figure a' is not a reflection of a, as there is no plane of symmetry between them. In type 22, the tetragonal alternating, where there are no planes of symmetry and the vertical axis is a tetragonal alternating axis, the etch figure a will be asymmetric ; and if revolved around the vertical axis c 90, then reflected over the equatorial plane, will become congruent with a 7 , on the adjacent prism face. In type 21, the tetragonal polar, where the vertical axis is a te- tragonal axis, the etch figure a will be asymmetric and will become congruent with a' by a rotation of 90 around the vertical axis c. From the several diagrams it will be seen that each of the seven possible prisms of the first order, though alike in outward form, possesses the symmetry of the type, which is revealed by the shape and relation of the etch figures on the form. Crystalline elements. In the isometric system, as the axes are all interchangeable, the crystalline characters are fixed and are the same for all substances crystallizing in the system. In the tetrag- onal system the axial ratio - varies with the substance ; its value is constant, however, for each chemically pure substance. The axial ratio is calculated from the angles of the fundamental forms, or those forms which intercept the axes at unity. TETRAGONAL SYSTEM 83 Example. The axial ratio of zircon is calculated directly from the pyramid of the second order (101), Fig. 147; the angle cao is found by measurement to be 32 38' 4", in the triangle coa right-angled at o. Tan cao = , but oc = c and oa = a ; tan ca cao = - = .6404. In the tetrag- a onal system the lateral axis a is assumed as the unit of meas- urement, therefore c = .6494 is the axial ratio of zircon. When the pyramid of the first order is the fundamental form in which the angle is measured (111), Fig. 148 at a', at right FIG. 147. Zircon, (101). angles to aai ; tan ca'o = oc oa 7 '' oc = oa' (tan ca'o), also oa' = (tan ca'o). In rutile ca'o is 42 10'. Log tan 42 UK = 9.959616 +10 Log jV2 = 1.849485 Log c = 9.809101 -10 c = .644 + , the axial ratio of rutile. FIG. 148. Cassiterite, (111). . When the -axial ratio is known, it is an easy problem to calculate the value of the variables m and n in any set of parameters ; thus in rutile there is a pyra- mid of the second order in which the angle corresponding to cao, Fig. 147, is 78 15' ; tan 78 15' = - = - = 4.8 or nearly 5 ; its pa- a i rameters would be (a : a : 5 c), and indices (501). CHAPTER V HEXAGONAL SYSTEM THE hexagonal system includes all those crystals which may be referred to four axes, three of which, the lateral axes, lie in one plane, the equatorial plane. They are equal and interchangeable and inclined 60 to each other. They are all designated by the let- a ter a. The order of + and extremities are as shown in Fig. 149. The fourth or c axis is the vertical axis and is at right angles to the a axes and not inter- changeable with them. It may be either longer or shorter. In the hexagonal system the parameters have four terms, and are written in the following order, nai : pa 2 : a 3 : me. As all three lateral axes are in the equatorial plane and all faces, except the base, in- tersect this plane in straight lines, two lateral axes and the c axis will fix, the in- clination of any face, for the straight line dd', Fig. 150, is fixed by the inter- cepts on the axes ai and a 2 . The value of the intercept a 2 , or the coefficient p in the general set of parame- ters, when one intercept is FIG. 149. a 84 FIG. 150. HEXAGONAL SYSTEM 85 unity p, is a function of the other intercept n. The value of p ex- pressed in terms of n is - ; - - will increase as n decreases, until n is unity, when n i becomes infinity, or if the value of n increases, decreases, and when n = 2, = 2 ; the value n i n i of n may vary between i as its minimum limit and 2 as its maxi- mum limit ; m, the coefficient of c, is independent of n and may vary between and oo . There will also be four terms in the in- dices of any plane, thus hkil, where i represents the smallest inter- cept. Of the three indices hki, two will always be of the same sign and the third of the opposite sign, and the algebraic sum of these three indices is always zero. Their relative values are i>h>k and h -j- k = i. In writing the indices 1 always stands last and represents the c axis not interchangeable with the lateral axes. Twelve of the thirty-two types are included in the hexag- onal system, all of which possess at least one axis of trigonal symmetry. CLASS, HEXAGONAL HOLOHEDRAL (HOLOSYMMETRIC) TYPE 20, DIHEXAGONAL EQUATORIAL Symmetry. Crystals of this type possess one dihexagonal axis, the c axis, 6 didigonal axes, all lying in the equatorial plane and inclined at an angle of 30 to each other, three of which are the FIG. 151. FIG. 152. Type 20, Dihexagonal Equatorial. 86 MINERALOGY lateral crystallographical axes. The remaining three bisect the angles between the lateral axes. There are seven planes of sym- metry, one of which, the equatorial plane, contains the didigonal axes and is at right angles to the c axis. The other six planes all intersect in the c axis and each contains one of the didigonal axes. They are therefore inclined to each other at an angle of 30, Fig. 151. These seven planes of symmetry divide space into 24 equal por- tions or solid angles. The largest number of faces on any hexag- onal form will be 24, or one face in each solid angle. There is also a center of symmetry and the forms of this type will all be bounded by pairs of parallel faces, Fig. 152. Forms I. Dihexagonal pyramid ; na : n i a : a : me ; (hkil) . This form is represented by one face in each of the 24 solid angles and is bounded by 24 scalene triangular faces, Fig. 153 ; each face FIG. 153. Dihexagonal Pyramid, na: a: a: me; (hkil). PIG. 154. Hexagonal Pyra- mid of the First Order, a : oo a : a : c, (hohl). cuts the lateral axes at a different distance. The equatorial edges are all equal, and the alternate polar edges are equal. II. Hexagonal pyramid of the first order ; a : oo a : a : me ; (hohl). If the potes in Fig. 152 be moved into the intermediate planes of symmetry so as to lie on that side of the triangle between the intermediate and the hexagonal axes, then the number of faces will be reduced to 12, and a new form will result, the hexagonal pyra- mid of the first order, Fig. 154, bounded by 12 isosceles triangles. HEXAGONAL SYSTEM 87 Each face cuts two of the lateral axes at an equal distance and is parallel to the third. The axes therefore end in the tetrahedral angles, making it a pyramid of the first order. III. Hexagonal pyramid of the second order; 2 a : 2 a : a : me ; (hh2hl) . If the poles in Fig. 152 are moved into the diametral planes, then the faces of the most general form will be reduced to 12 isosceles triangles, Fig. 155, each face cutting two of the lateral axes at an equal distance and the third at one half that distance. The a axes will bisect the T FIG. 155. Pyramid of the equatorial edges, making the form a pyra- second Order, 2 a : 2 a : a : c, mid of the second order. (hh2hi). IV. Dihexagonal prism; na : n i a : a: ooc; (hkio). The poles are now moved to the equatorial plane between the axes of symmetry, when the faces will be reduced to 12, all of which are FIG. 156. Dihexagonal Prism, na : ^ a : a : me, (hklo). FIG. 157. Hexagonal Prism of the First Order, a : oo a : a : oo c, (hoho). parallel to the c axis, yielding an open form, Fig. 156, the dihex- agonal prism, alternate edges of which are similar. 88 MINERALOGY V. Hexagonal prism of the first order ;< a : oo a : a : oo c ; (ho hi). Three possible positions of the poles now remain, the three angles of the triangle, each position yielding one of the three fixed forms. The four forms already developed represent the variable forms, there being a series of each. If the poles coincide with the intermediate axes, Fig. 152, four faces of the most general form will fall in one plane, producing an open form, the hexagonal prism of the first order, bounded by 6 faces, all of which are parallel to the c axis, Fig. 157. Each face cuts two lateral axes at the same distance and is parallel to the third. VI. Hexagonal prism of the second order ; 2 a : 2 a : a : oo c ; (hh^ho). In this form the poles will coincide with the lateral crystallo- graphical axes. It is bounded by 6 faces, each of which cuts two lateral axes at the same distance, and the third at one half that distance. The axes will therefore terminate in the center of the faces, Fig. 158. VII. Basal pinacoid ; oo a : oo a : oo a : me, (oooi). If the poles are moved to the c axis the number of faces will be reduced to a single pair of faces parallel to the equatorial plane. They terminate the prisms as shown in Fig. 158. FIG. 158. Hexagonal Prism of the Second Order (hh2ho). * f FIG. 159. Beryl, a Combi- nation of m (10TO), u (2021), s (1121), p(10ll),c (0001). The forms possible to combine on crystals of the dihexagonal equatorial type are : HEXAGONAL SYSTEM 89 Dihexagonal pyramid, (hkil). Hexagonal pyramid of the first order, (hohl). Hexagonal pyramid of the second order, (hh2hl). Dihexagonal prism, (hkio). Hexagonal prism of the first order, (hoho). Hexagonal prism of the second order, (hh2hb). Hexagonal base, (0001). Examples. Few minerals crystallize in this type; as a rule the holosymmetric class in other systems is the most important class in the system. Beryl, Be 3 Al 2 (Si0 3 )6, Fig. 159, repre- sents a combination of five forms on a crystal of beryl. Hanksite, 9 Na 2 SO 4 . 2 Na 2 C0 3 . KC1, FIG. 160. Hanksite, c(0001), m(1010),p(10Tl). Fig. 160, represents a combination of three forms on a crystal of hanksite. CLASS, RHOMBOHEDRAL HEMIHEDRQNS TYPE 19, DIHEXAGONAL ALTERNATING Symmetry. Crystals of this type possess one dihexagonal al- ternating axis, the c axis; three didigonal axes, the lateral crys- tallographical axes, three planes of symmetry, intersecting in the c axis and each containing one of the intermediate lateral axes, and FIG. 161. Type 19, Dihex- agonal Alternating. FIG. 162. The Positive Scale- nohedron, k (hhfl). also a center of symmetry, Fig. 161. It is to be noted here that the equatorial plane of type 20 is lost and the equatorial edge in the two new forms of the type is represented by a zigzag edge. Forms of this type may be derived from the holohedral forms of type 20 90 MINERALOGY by the extension of all planes in alternate dodecants above and below the equatorial plane, as the shaded faces of Fig. 162. Forms n na : a : a : me I. Scalenohedron n ~ J . ; K(hkil), K(khil). 2 The faces represented by the poles in Fig. 161 when extended will yield the minus scalenohedron, Fig. 163, a form bounded by 12 similar scalene triangles. Six faces are grouped around the extremities of the c axis. Alternate edges are equal both as to length and angle. The equatorial edge of the dihexagonal pyramid is replaced by 6 equal zigzag edges, each of which is bisected by the ex- tremity of a lateral axis. II. Rhombohedron of the first order, ; K(hohl), FIG. 163. The Minus Scaleno- hedron, K (hkfl). K(ohhl). When the poles of Fig. 161 are moved into the planes of sym- metry, a form, the rhombohedron of the first order, Fig. 164, bounded by 6 simi- lar rhombic faces, is the result. The form is plus if the pole is moved into the plane of symmetry lying between the ai and a 3 axes in front, and the minus form is pro- duced when they are moved into the plane between a 2 and a 3 . There are three faces grouped around each ex- tremity of the c axis, and three equal edges. The six zigzag edges which are bi- sected by the ex- FIG. 164. The Minus Rhom- , ... ' - ,, FIG. 164 a. The Rhombo- bohedron of the First Or- 1 hedron of the Middle Edge der, K (ohfil). . lateral axes are and Scalenohedron. HEXAGONAL SYSTEM 91 FIG. 165. Combination of the Scalenohedron and the Rhombohedron of the Mid- dle Edge. equal in length to the polar edges but differ in angle. These edges correspond in position to the zigzag edges of the scaleno- hedron. For each scalenohedron there is a corresponding rhombohedron; Fig. 164 a, termed the rhombohedron of the middle edges. When in combination, it bevels symmetrically the edges of the scalenohedron, Fig. 165. Other forms. All other possible po- sitions of the poles yield forms similar in shape to the holohedral forms of type 20, but when found in combination with a rhombohedron or a scalenohedron they must be considered as of hemihedral sym- metry. The possible forms to combine in this type are : Plus and minus scalenohedron, K (hkil), K (khil). Plus and minus> rhombohedron of the first order, K (hohl) , K (phhl) . Hexagonal pyramid of the second order, (hh2hl). Dihexagonal prism, (hkio). Hexagonal prism of the first order, (hoho). Hexagonal prism of the second order, (hhiho). Hexagonal base, (0001). Examples. The rhombohedral class is the most important class of the hex- agonal system, as a large num- ber of common and commer- Pf minerals belong here. Calcite, CaCO 3 , Fig. 166, is a com- FIG. 167. Hematite, c(oooi), FIG. 166. Calcite, Com- bination of R (1011), m(ioro), v(2i3i). tant bination of R(lOll), m(10lO), v(2131). 1} ' e Hematite, Fe 2 O 3 , Fig. 167, is a combination of a base and the plus and minus rhombohedron. Corundum, A1 2 3 ; Siderite, FeC0 3 ; Arsenic; Antimony; Brucite, MgO . H 2 O, also crystallize in this type. 92 MINERALOGY CLASS, DIHEXAGONAL HEMIMORPHIC TYPE 18, DIHEXAGONAL POLAR Symmetry. By a polar development of the holohedral forms of type 20, all the symmetry lying in the equatorial plane is lost, leaving the dihexagonal axis and the six vertical planes intersecting in the c axis as the symmetry of this type, Fig. 168. Forms. The new forms would be, upper and lower dihexagonal hemi- pyramids, Fig. 169. Also upper and lower hexagonal hemipyramids of the first and second orders, Fig. 170. Possible forms to be found in com- FIG. 168. Type is, Dihexag- bination on crystals of this type would onal Polar. TL Upper and lower dihexagonal hemipyramid, na : : a : me n i ; (hkfl), (hkil). Upper and lower hemipyramid of the second order, ,./^2a:2a: a:mc u/ v ~^~ FIG. 169. The Upper Dihex- agonal Plemipyramid, (hkH.) FIG. 170. The Upper Hex- agonal Hemipyramid of the First Order, (hofil). Upper and lower hemipyramid of the first order, u /,(ii^iM). (hoSl) , (hoS) . Dihexagonal prism, na : : a : oo c ; (hkio) . Hexagonal prism of the first order, a : oo a : a : oo c ; (1010) Hexagonal prism of the second order, 2a:2a:a:ooc; (i 120) . HEXAGONAL SYSTEM 93 Upper and lower base, ooa: ooa: ooa: c; (oooi), (oooi). Examples. Greenockite, CdS, Fig. 171, is a combination of five forms. Iodide of Silver, Agl, Fig. 172, is a top-shaped combination of an upper and a lower hemipyramid with the first order prism. FIG. 171. Greenockite, a Combination of p (lOTl), p (1011), m (101 0), z (2021), c(OOOT). FIG. 172. m (1010), p(iori), z (2021). Wurtzite, ZnS, and Zincite, ZnO, are other minerals which crystallize in this type. CLASS, HEMIHEDRAL PYRAMIDAL (PARALLEL-FACED HEMIHEDRONS) TYPE 17, HEXAGONAL EQUATORIAL Symmetry. Crystals of this type are symmetrical in regard to one axis of hexagonal symmetry, the c axis, one plane of sym- FIG. 173. Type 17, Hexagonal Equatorial. FIG. 174. 94 MINERALOGY metry, the equatorial plane, and a center, Fig. 173. The forms are parallel-faced hemihedrons, and may be considered as derived from the holohedrons of type 20 by extending alternate pairs of faces which intersect in the equatorial edge, as the shaded faces in Fig. 174:, which will produce a minus hemihedron. Forms I. Hexagonal pyramid of the third order, na: a: a: me n i ; -Tr(hkil), ir(khil). The faces represented by the poles of Fig. 173 bound a pyramid, which in shape does not differ from the hexagonal pyramid of the first or second order except here the lateral axes do not terminate either in the center of the faces or bisect the edges, but on the line \ FIG. 175. The Hexagonal Pyra- mid of the Third Order. FIG. 176. connecting these two points, Fig. 175, also Fig. 176, which is a plan of the first, second, and third order pyramids and prisms drawn on the equatorial plane. II. Hexagonal prism of the third order, na: a:a: ooc ; ir(hkio), ir(khio). If the poles in Fig. 173 are moved so as to lie upon the equator, between the a axes and the intermediate axes, then two faces of the HEXAGONAL SYSTEM 95 pyramid of the third order will fall in one plane, producing a new open form, the prism of the third order, Fig. 177, in which the a axes neither terminate in the edges or in the center of the faces, but on the line drawn between these two points. III. Other forms of this type are like the hexagonal holohedral in shape. The possible forms to be found in combination will be : Plus and minus hexagonal pyramid of the third order, Tr(hkil), ir(khil). Hexagonal pyramid of the first order, Tr(hohl). Hexagonal pyramid of the second order, ir(hh2hl). Plus and minus hexagonal prism of the third order, ir(hkio), ir(khio). Hexagonal prism of the first order, Tr(hoho). Hexagonal prism of the second order, ir(hh2ho). Hexagonal base, ir(OOOl). Examples. Apatite, Ca5(FCl)(PO 4 ) 3 , Fig. 178, shows a combina- tion of the three pyramids, a prism, and the base. Pyromorphite, Pb 5 Cl(PO 4 ) 3 ; Mimetite, Pb 5 Cl(PO 4 )3, and Vana- dinite, Pb 5 Cl(PO 4 )3, also crystallize in this group. FIG. 177. Hexagonal Prism of the Third Order. FIG. 178. Apatite, a Combination of p (1011), u (1231), s (1121), m (1010). CLASS, TRAPEZOIDAL (PLAGIOHEDRAL) HEMIHEDRONS TYPE 16, HEXAGONAL HOLOAXIAL Symmetry. Crystals of this type possess all the axes of the di- hexagonal equatorial type, but no planes, or center of symmetry. They have therefore one axis of hexagonal symmetry, the c axis, and six digonal axes corresponding to the lateral and intermediate axes, lying in the position of the equatorial plane, Fig. 179. The forms are plagiohedral and may be derived from the holohedral forms by extending alternate faces around the poles, Fig. 180. 96 MINERALOGY Forms. Hexagonal trapezohedrons, na: a : a : me r/1 n i r(hkil), T(khil). ', O : The faces represented by the poles in Fig. 179 yield a form bounded by 12 similar trapezoidal faces, Fig. 181, the right trape- ..--... zohedron. If the shaded faces are ex- tended, the left trapezohedron, Fig 180, is formed. Six faces are grouped around each extremity of the c axis, making equal angles and equal polar edges. The median edges are alter- nately long and short; the crystallo- graphical axes terminate in the middle of the long edges. In look- ing at a south polar edge, with the crystal form vertical, if the long median edge is to the right, it is a right-handed form ; if to the left, it is a left-handed form. Other forms. All other possible positions of the poles, as the sides and angles of the triangles in Fig. 179, will yield holohedral FIG. 179. Type 16, Hexagonal Holoaxial. FIG. 180. The Left Hexagonal Trapezohedron. FIG. 181. The Right Hexago- nal Trapezohedron, r (hkil). shapes. The possible forms therefore to be found in combination on crystals of this type will be : Right and left hexagonal trapezohedrons, r(hkil), r(khil). Hexagonal pyramid first order, T(hohl). Hexagonal pyramid second order, T(hh2hl). HEXAGONAL SYSTEM 97 Dihexagonal prism, r(hkio). Hexagonal prism first order, r(hohl). Hexagonal prism second order, r(hh2ho). Hexagonal base, r(OOOl). Examples. There has been as yet no mineral assigned to this type ; in fact, the trapezohedron has never been observed on any crystal. There are, however, several salts included here, as barium stibiotartrate, Ba(C4H 4 6 )2SbO 2 , KN0 3 , which from the symmetry of its etching figures must crystallize with an hexagonal holoaxial symmetry. CLASS, RHOMBOHEDRAL TETARTOHEDRONS TYPE 15, HEXAGONAL ALTERNATING Symmetry. Crystals of this type '* possess an alternating hexagonal axis, ..-'' \ xao / "\ the c axis, and a center of symmetry, / \ / <3, but no planes of symmetry, Fig. 182. / The forms of this type are tetarto- ,/_ Ol V' hedrons, derived by superimposing 1 / \ * / type 17 upon type 19, and extending \ the faces not selected by these two \ x 24 / types to produce a new form. If the ? \ ..'' dihexagonal pyramid is rolled out, the faces being numbered ; the equa- FlG - 182. Type 15, Hexagonal torial edge represented by a horizon- tal line, and the terminations of the a axes marked by a vertical line, as here represented : a 3 a 2 ai t 16 #U9 8 20 9 10 n n n_ n 23 24 FIG. 183. The Plus Rhombohedron of the Third Order, (hkil). H If a line be drawn through those faces which are extended by the method of selec- tion used to produce forms of type 19, and a line drawn under those selected in type 17, then it will be seen that there are three faces above and three faces below the equa- torial plane not marked. The faces 2, 6, and 10 do not lie sym- metrically above the three faces, 16, 20, and 24, neither do these faces lie symmet- 98 MINERALOGY rically between the lateral axes. When these six faces are extended, the plus right tetartohedron will be formed, but it is easily seen that the superposition of the two hemihedrons may be so arranged that in place of 2 being selected, 1 and the corresponding faces could have been taken, forming the plus left rhombohedron or 13 could have been selected, the minus right ; or 14, the minus left, thus yielding four possible forms, the plus and minus right, congruent forms, and the plus and minus left, also congruent forms. The rights are not congruent with the lefts. Forms I. Rhombohedrons of the third order, r/l na: a : a : me n i ; +r(Uril), H-l(ikhl), - r(khil), - l(ihkl). When the faces represented by the poles of Fig. 182 are extended, they will inclose space and yield a rhombohedron of the third order, Fig. 183, which does not differ in shape from the rhombohedron of the first order; the lateral axes, however, do not end in the central point of the edges, but terminate asymmetrically in the faces on the line drawn between the central points of the zigzag edges. As is shown above, there are four rhom- bohedrons of the third order. Fig. FIG. 184. The Plus Left .Rhom- bohedron of the Third Order, (ikfil). 183 is a plus right and Fig. 184 is a plus left rhombohedron of the third order. II^Rhombohedron of the second order f 2&: 2a - : a:mc \ (hhffl), ( 2 hhhl). If the poles in Fig. 182 are moved into the diametral planes, it will be the same as revolving the rhombohe- dron of the third order until the lateral axes terminate in the median point of the line drawn between the centers of the zigzag edges. In this position the right form will coincide with the , loft- OYI^ Ur Y j FlG - 185 - Rhombohedron of left, and only phis and minus forms the Second Order, (hh 2 hi). HEXAGONAL SYSTEM 99 will remain. These are the rhombohedrons of the second order, Fig. 185, derived from the hexagonal pyramid of the second order. III. Rhombohedrons of the first order. If the poles are moved into the plane containing the intermediate axes, the resulting form is the rhombohedron of the first order. The three rhombohedrons differ only in the position of the lateral axes : in the first order they end in the central point of the zigzag edges ; in the second order they end in the median point of the line connecting the central points of the zigzag edges ; in the third order they end asymmetrically on this same line between the above two points. Other forms. When the poles are moved to the equatorial plane the first, second, and third order prisms are formed. The possible forms to combine on crystals of this type will be : Plus and minus right and plus and minus left_rhombohedrons of the third order, (khil), (hkil), (ihkl), (ikW). Plus and minus rhombohedrons of the sec- ond order, (hhThl), (2hhhl). Plus and minus rhombohedrons of the first order, (hkil), (khil). Plus and minus prisms of the third order, (hoho), (ohho). Hexagonal prism of the second order, (hh2ho). Hexagonal prism of the first order, (hoho). Hexagonal base, (0001). Examples. Dolomite, MgCa(CO 3 ) 2 ; Phenacite, Be 2 Si0 4 ; Willemite, Zn 2 SiO 4 ; and Dioptase, H 2 CuSi0 4 , crystallize in this type, Fig. 186. CLASS, PYRAMIDAL HEMIMORPHIC TYPE 14, HEXAGONAL POLAR Symmetry. Crystals of this type possess an axis of hexagonal symmetry, the c axis, and no plane or center of symmetry. It is a polar development of the hexagonal equatorial type. Fig. 187 shows the symmetry. FIG. 186. Phenacite, a Combination of the Negative Right Rhom- bohedron of the Third Order and the Prism of the First Order. 100 MINERALOGY Forms I. Hexagonal hemipyramid of the third order, u/l na: a:a:mc n i ; (hkil), (khil), (hkil), (khil). The upper or lower half of the pyramid of the third order may occur independently, Fig. 188. Other forms are the same as in type 18. The possible forms to combine in this type will be Plus and minus upper and plus and minus lower hemipyramids of the third order, (hkil), (khil), (hkil), (khil). / o o .' o ,- FIG. 187. Type 14, Hexagonal Polar. FIG. 188. The Upper Hexagonal Hemipyramid of the Third Order. Upper and lower hemipyramid of the first order, (hkil), (hkil). Upper and lower hemipyramid of the second order, (hhzhl), (hhahi). Plus and minus hexagonal prism of the third order, (hkio), (khio). Hexagonal prism of the first order, (1010). Hexagonal prism of the second order, (1120). Hexagonal base, upper and lower, (0001), (OOOl). Examples. Crystals in this type are rare and the hemipyramid of the third order, which is the only form characteristic of the type, does not occur on any min- eral, but from the symmetry of the etch- ing figures, nephelite, KoNaeAlgSiaO^, is FIG. 189. Combination of ,, , p(ion) p(ioil m(ioTo) P lace d here; as are also the double sul- c(oooi), c'(oooi). phate of the alkali metals, potassium and HEXAGONAL SYSTKM '1.01 lithium, KLiSO4 ; lithium, ammon um, LiNH 4 SO 4 ; and lithium and rubidium, LiRbSO 4 . Fig. 189 shows the appearance of these combinations. CLASS, TRIGONAL HEMIHEDRONS TYPE 13, DITRIGONAL EQUATORIAL Symmetry. Crystals of this type possess one ditrigonal axis, the c axis, three didigonal axes, the intermediate lateral axes, and four planes, three -of which intersect in the c axis and each con- tains one of the didigonal axes. The fourth is at right angles to these and contains the a axes. Fig. 190 illustrates this sym- FIG. 190. Type 13, Ditrigonal Equatorial. FIG. 191. The Plus Di- trigonal Pyramid (hkll). metry. The forms may be considered as hemihedrons, derived from type 20 by extending all the faces in alternate dodecants around the north pole and dodecants below, which intersect with these in the equator, as the shaded faces in Fig. 191. Forms I. Ditrigonal pyramid, na: a : a : me n i (hkil), (ihkl). The faces represented by the poles in Fig. 190 or shown in Fig. 191, in their relation to the hexagonal pyramid, when extended, form the plus ditrigonal pyramid. A form bounded by 12 scalene triangles, meeting in six equal equatorial edges. There are 12 polar edges, six around each extremity of the c axis, alternate edges are similar both as to length and angle. Fig. 192 represents the ditrigonal pyramid. 10'.' MINERALOGY a : oo a : a : me II. Trigonal pyramid of the first order, f - J ; (hohl), (ohhl). When the poles of Fig. 190 are moved into the vertical planes of symmetry, two adjacent faces of the ditrigonal pyramid will fall in one plane, producing a form, the trigonal pyramid of the first order, bounded by six isosceles triangles, Fig. 193; here the rela- FIG. 192. The Negative Di- trigonal Pyramid. FIG. 193. Trigonal Pyra- mid of the First Order, (Olll). tion of the faces to the hexagonal pyramid of the first order is also shown. The lateral crystallographical axes terminate, two in each equatorial edge, dividing it into three equal parts. n III. Ditrigonal prism, na: n i a:a: oo c ; (Mdo), (khio). If the poles are moved to the equator between the a axes and the intermediate axes, the ditrigonal prism results, Fig. 194, a form > "~~ ^ j ' -. '" ,...- J - FIG. 194. Ditrigonal Prism, (hkll). FIG. 195. The Trigonal Prism of the First Order, (ohho). HEXAGONAL SYSTEM 103 bounded by six faces. Alternate solid angles are equal, three being less than 120 and three greater. The crystallographical axes bisect the edges. IV. Trigonal prism of the first order, (hoho), (ohho). When the poles are moved on the primitive circle to coincide with the didigonal axes, the resulting form is the trigonal prism of the first order, Fig. 195, bounded by three equal faces, the lateral axes terminating, two in each face, as indicated in Fig. 195. Other forms. Other possible positions of the poles will produce apparent holohedral forms, i.e. the hexagonal pyramid and prism of the second order and the base. Forms possible to combine on crystals of this type will be Plus and minus ditrigonal pyra- mids, (hkil), (ihkl). Plus and minus trigonal pyramids of the first order, (hohl), (ohhl). Hexagonal pyramid of the sec- ond order, (hh2ho). Plus and minus ditrigonal prisms, (hkio), (ihko). Plus and minus trigonal prisms of the first order, (hoho), (ohho). Hexagonal prism of the second order, (hh2ho). Hexagonal base, (0001). Example. There is only one example of a substance crystalliz- ing in this type, the mineral benitoite, BaTiSiaOg, Fig. 196. CLASS, DITRIGONAL HEMIMORPHIC TYPE 12, DITRIGONAL POLAR Symmetry. The c axis is an axis of ditrigonal symmetry, which is also polar, with three planes of symmetry intersecting in it. The forms are derived from the ditrigonal equational type by a polar development of the c axis, Fig. 197. Forms I. Ditrigonal hemipyramids, n FIG. 196. Benitoite, Combina- tion of p(10Tl), p'(Olll), m(ioro), r(10T2), c(0001). u/1 na: a : a : me n i ; (hkil), (ikhl), (hkil), (ihkl). 104 MINERALOGY Fig. 198 represents that portion of the ditrigonal pyramid above the equatorial plane. In this hemipyramid the lateral axes a ' FIG. 197. Type 12, Ditrigonal Polar. FIG. 198. The Upper Plus Ditrigonal Hemipyramid (hkil). hold the same relation to the edges as in the ditrigonal equatorial types. II. Trigonal hemipyramids of the first order, ; (hohl), (ohhl), (hohl), (ohhl). The trigonal hemipyramid appears here as a new form ; Fig. 199 represents the upper minus trigonal hemipyramid of the first order. Other forms in ' the type are simi- lar to the forms of the. ditrigonal equatorial, except the hexagonal FIG. 199. Upper Negative pyramid of the Trigonal Hemipyramid of seC ond Order and the First Order, (ohfil). , , the base are upper and lower forms which have appeared in the dihexagonal polar type. Combinations. The possible forms to combine in this type are : Upper and lower, plus_and minus ditrig- ona_l hemipyramid, (hkil), (khil), (hkfi), (khil). Upper and lower, plus and minus FIG. 200. Tourmaline, a Combination of the Upper and Lower Trigonal Pyra- mids r ; Hexagonal Prism, Second Order a; the Lower Base c, and the Tngonal Prism, First Order m. HEXAGONAL SYSTEM 105 trigonal hemipyramid of the first order, (hohl), (ohhl), (hohl), (ohhl). Upper and lower hexagonal hemipyramid of the second order, (hh2hl), (hhihf). Plus and minus ditrigonal prism, (hkio), (khio). Plus and minus trigonal prism of the first order, (hoho), (ohho). Hexagonal prism of the second order, (hh2ho) . Upper and lower base, (0001), (0001). Example. The common and important mineral tourmaline belongs to this type ; Fig. 200 represents a combination of forms as found on this mineral. CLASS, TRIGONAL TETARTOHEDRAL TYPE 11, TRIGONAL EQUATORIAL Symmetry. Crystals of this type have an axis of trigonal symmetry, the c axis, and one plane of symmetry perpendicular to it, Fig. 201. The class may also be considered as te- tartohedral derived by superposing type 19, scalenohedral hemihedral, upon type 13, trigonal hemihedral. Forms I. Trigonal pyramids of .the third FIG. 201. Type 11, Trigonal Order, Equatorial. r/l na : a : a : me n i ; (hkil), (khil), (ikhl), (ihkl). The faces represented by the poles in Fig. 201 bound the right plus trigonal pyramid of the third order, a form having six isosceles triangular faces. The a axes terminate asymmetrically in the equa- torial edges, as represented in Fig. 202. There are four pyramids of the third order : the plus and minus right, two congruent forms ; the plus and minus left, also two congruent forms. The rights and lefts are enantiomorphic. Fig. 203 is a minus left form. 106 MINERALOGY FIG. 202. Diagram of the Equatorial Plane, showing the Relation of the Trigonal Prisms and Pyramids to the Lateral Axes. 2 a: 2 a: a: mc\ II. Trigonal pyramids of the second order, (hh2hl), (2hhhl). When the poles in Fig. 201 are moved so as to lie on dotted line representing the the FIG. 203. The Minus Left Trigonal Pyra- mid of the Third Order. lateral axes, a new form, the trigonal pyramid of the second order, is the result, Fig. 204 ; a form which will not differ from the trigonal pyramids of the third or first orders in appear- ance, but differs in its relation to the lateral axes, which ter- minate in the central point of each equatorial edge and in the opposite angle, as is shown in Fig. 202. III. Trigonal prisms of the third order, r/l na : - a : a : oo : c n i ; (hMo),(khlo),(ikho),(ihko). When the poles in Fig. 201 lie on the primitive circle, between the points a ; the terminations of the lateral axes and the points p, the HEXAGONAL SYSTEM 107 terminations of the intermediate axes, a new form, Fig. 205, the trigonal prism of the third order, will result. When the pole is to FIG. 204. The Positive Trigonal Pyramid of the Second Order, (hhahl). FIG. 205. The Negative Left Trigonal Prism of the Third Order, (ikEl). the right of the point p, it is a right form; when to the left of p, it is a left form; when between a and a 3 , a plus, and between a 3 and a 2 , a minus form. IV. Trigonal prisms of the second order, (hhlho), (2hhho). When the poles are at the points a, Fig. 201, a new form, Fig. 206, the trigonal prism of the second order, will result, in which the lateral axes terminate in the central point of the face and bisect the opposite edges, as illustrated in Fig. 206. The relation of the trigonal pyramids and prisms to the lateral axes is shown in Fig. 202, which is a plan of the equatorial plane. Other forms. All other positions of the poles will yield forms of the ditrigonal equatorial type. The possible forms to combine in this type will be : Right and left plus and minus trigonal pyramids_of the third order, (hkil), (khil), (ihkl), (ikhl). , . x . .j r ,i FIG. 206. The Plus Trig- Plus and minus_tngonal_pyramids of the onal Prism of the Second second order, (hhihl), (2hhhl). Order, (hhifio). 108 MINERALOGY Plus and minus trigonal pyramid of the first order, (hotil), (ohhl). Right and left plus and minus trigonal prisms of the third order, (hkio), (khio), (ihko), (ikho). Plus and minus trigonal prisms of the second order, (hh2ho), (2hhho). Plus and minus trigonal prisms of the first order, (hoho), (ohho). Base, (0001). Examples. As yet there are no representatives of this type. CLASS, TRAPEZOHEDRAL TETARTOHEDRAL TYPE 10, TRIGONAL HOLOAXIAL Symmetry. Crystals of this type are symmetrical in regard to one trigonal axis of symmetry, the c axis, and three digonal axes, the a axes. Fig. 207 illustrates the symmetry of the type. It may also be considered as a tetartohedral class, and the forms are derived by superimposing the rhombohedral hemihedrons, type 19, upon the trapezohedral hemihedrons, type 16, and extend- ing the faces not thus marked, as below : 2 6 10 $^Z J _ 6 . J>^ 10 ~ ~ 23 24 ' 15 19 23 In the rhombohedral method of selection, alternating dodecants above and below the equator are crossed out and suppressed ; in the ....... trapezohedral method every other .*' ^^ face above and below the equator is * / \ crossed out, as suppressed. There \ will remain of the 24 faces of the o \ / \ dihexagonal pyramid, represented \'" * " ! above, 6 faces, 2, 6, and 10 above, / \ / and 15, 19, and 23 below the equator. \ x When this method of selection is o \ /' compared with the rhombohedral *^...._ ,,-^' tetartohedral method, page 97, it FIG. 207. Type 10, Trigonal ^ U be S6en that there the faces be " Holoaxiai. low lie symmetrically between those above, but here they are not sym- metrically located, as 15 is nearer 2 than to 6, which in the new form will produce a long inclined edge between 6 and 15 and a short edge between 2 and 15. These corresponding edges are equal HEXAGONAL SYSTEM 109 in type 15, forming the rhombohedron of the third order. As in all other tetartohedral classes, there are here also four new forms. In the selection above, face 2 and those corresponding are taken, forming the plus right form. The selection may be so arranged that 1, or 13, or 14, and corresponding faces should be selected ; 2 and 13 are the plus and minus right congruent forms and 1 and 14 are the plus and minus left congruent forms. By applying the method as above in turn to each of the holohedral forms of type 20, the new forms of this type will be produced. Forms I. Trigonal trapezohedrons, r/1 na: a : a : me n i KT(hkil), KT(khil),KT(ihkl), KT(ikhl). From the dihexagonal pyramid the four trigonal trapezohedrons are derived, of which the plus right is represented in Fig. 208, and the plus left in Fig. 209. There are six equal polar edges, three at each pole, three long and three short median zigzag edges, which are bisected by the terminations of the a axes. II. All other positions of the poles in Fig. 207 will yield forms already described, as the trigonal trapezohedron is the only new form of the type. The possible forms to com- bine in the type are : Plus and minus right and left trigonal trape- PIG. 208. -The Positive zohedrons, KT(hkll), KT(klnl), KT (ihkl), KT (ikhl). Plus and minus rhombohedron of the first order, KT (hohl), (ohhl). Plus and minus jtrigonal pyramid of the sec- ond order, KT (hh2hl), KT (ihhhl). Plus and minus right and jeft ditrigonal prisms, KT (hkio) , KT (khio) , KT (ihko) , KT (ikho). Plus and jninus trigonal prisms second FIQ 2Q9 _ The plus Order, KT (hh2ho) , KT (2hhho) . Left Trapezohedron. Right Trigonal Trape- zohedron. 110 MINERALOGY Hexagonal prism first order, KT (hoho). Base, (0001). Examples. Quartz, SiO 2 , crystallizes in this type, of which Fig. 210 is a combination of the hexagonal prism, first order, plus FIG. 210. Right-handed Quartz : Combination of m(10lO), r(1011), z(OlTl), , x(5161). FIG. 211. Lef t-hand_ed Quartz, m(ioro), r(10n), z (Olll), 8(2111), x(65Tl). and minus rhombohedrons, first order, plus trigonal pyramid, and the right plus trigonal trapezohedron, a right-hand crystal. Fig. 211 is a left-hand crystal. Cinnabar, HgS, also belongs here. CLASS, TRIGONAL HEMIMORPHIC TYPE 9, TRIGONAL POLAR *">.. Symmetry. Crystals of this type possess one axis of trigonal sym- metry, the c axis, Fig. 212. The type is a polar development of the trigonal equatorial, in which the hemitrigonal pyramids may occur independently, yielding two new forms. FIG. 212. Type 9, Trigonal Polar. na: :a:mc r/1, u/1 n i Forms I. Trigonal hemipyramids of the third order, ; (Uril), (khtt), (ikhl), (ihkl), (hkU), (khil), (ikhl), (ihkl). HEXAGONAL SYSTEM 111 Derived from the trigonal pyramid of the third order there are eight trigonal hemi- pyramids, of which Fig. 213 represents the upper left minus hemipyramid, and the poles in Fig. 212 are of the upper right plus form. II. Trigonal hemipyramids of the second order, FIG. 213. The Upper Left Negative Trigo- nal Hemipyramid of the Third Order. 11/1 2a: 2a: a: me (hh2hl), (hh2hl), (2hhhl), ( 2 hhhl). There are four trigonal hemipyramids of the second order, de- rived from the trigonal pyramid of the second order, all of which may occur independently. Fig. 214 is the upper minus trigonal hemipyramid of the second order. The forms possible to combine in this type are as follows : Plus and minus right and left upper and lower hemipyramids of the third order, (hkil) , (khil) , (ihkl) , (ikhl) , (hkii) , (khil) , (ihkl) , (ikhl) . FIG. 214. The Upper Negative Trigonal Hemi- pyramid of the Second Order. FIG. 215. Combination of Forms showing the Polar Development of Crystals of Sodium Periodate. Plus and minus upper and lower trigonaHiemipyramids of the second order, (hhihl), (2hhhl), (nh2HT), (2hhhT). Plus and minus upper and lower trigonal hemipyramids of the first order, (hohl), (ohhl), (hohl), (ohhl). Plus and minus right and left trigonal prisms of the third order, (hklo), (ihko"), (khio), (ikho). Plus and minus trigonal prisms of the second order, (hoho) , (ohho) . Plus and minus trigonal prisms of the first order, (hh2ho), (2hhho). Upper and lower base, (0001), (OOOl). 112 MINERALOGY Example. Sodium periodate, NaKX, 3 H 2 O, crystallizes in trig- onal hemipyramids. Fig. 215 is a combination of two plus upper hemipyramids of the second order and one of the first order with the lower base. Crystalline Characters Like the tetragonal system, the hexagonal system has but one variable crystalline character, the axial ratio, - . This is calcu- lated in a similar way, by means of the angle between the basal pinacoid and the pyramid face of the first or second order. I. When the pyramid of the second order is used, - = tan (oooi) A (ii22). Example. In the mineral beryl, the angle between the normal to the base and that of the unit pyramid of the second order is 26 31', - = 0.4988+, and as a = 1, c = 0.4988+, which is the unit a of measurement on the c axis. II. When the pyramid of the first order is used, - = tan (0001) A (1011) X 1/2V3. a Figure 216, CO = c and OA = a = i. ^rk tan CDO = CO = tan CDO X DO. In the triangle aDO, right- angled at D, and DOa = 30, DO = l/2V3,then c = tan CDO X 1/2V3, but the angle CDO is equal to the angle between the poles of the base and the unit pyramid of the first order, (0001J011). In the mineral beryl the angle 0001J011 is 29 56', c = tan 29 56' X 1/2V3. Log tan 29 56' = 9.760272 + 10 ' Log 1/2V3"= 1.937530 Log c + 10 = 9.697802 c= .4988+ CHAPTER VI THE ORTHORHOMBIC, MONOCLINIC, AND TRICLINIC SYSTEMS THE ORTHORHOMBIC SYSTEM CRYSTALS of this system possess three crystallographical axes; all at right angles, none of which are interchangeable. The vertical axis is represented by c. The longer lateral axis or macro-axis is represented by b and is placed horizontally from right to left, while the short or brachy-axis, a, is at right angles to b. Included in the system are three types all of which have at least one axis of digonal symmetry. The forms fall into three groups, according to the relation of their faces to the axes. If the faces cut all three axes, it is a pyramid and there will be no in its indices, as here there are no pyramids or prisms of the second order ; if the face cuts two axes and is paral- lel to the third, it is a prism and there will be one in its indices ; when parallel to a lateral axis it is a dome and receives the name of the lateral axis to which it is parallel, as macrodome. A dome is a prism parallel to a lateral axis. When the face is parallel to two axes, it is a pinacoid and there will be two zeros in its indices ; when parallel to the lateral axes it is a basal pinacoid ; when parallel to c and one of the lateral axes, it takes the name of the lateral axis to which it is parallel, as brachypinacoid. CLASS, ORTHORHOMBIC, HOLOSYMMETRIC, OR HOLOHEDRAL TYPE 8, DIDIGONAL EQUATORIAL Crystals of this type possess three didigonal axes, correspond- ing to the crystallographical axes; three planes, the diametral planes, and a center of symmetry, Fig. 217. The largest number of faces possible on any crystal form of the type will be 8, one in each octant into which the three planes of symmetry, Fig. 218, divide space. i 113 114 MINERALOGY Forms I. Orthorhombic pyramids, na : b : me; (hkl). The poles in Fig. 217 represent the orthorhombic pyramid; it is bounded by 8 similar scalene triangular faces, which inclose space, FIG. 217. Type 8, Didigonal Equatorial. FIG. 218. The Orthorhombic Axes and Planes of Symmetry. Fig. 219. The crystallographical axes terminate in the tetrahedral angles. There are three series of pyramids : a. The unit series, & : b : me, (hhl), where the variable lies on the c axis ; Fig. 220 represents the unit series of pyramids. FIG. 219. The Unit Pyramid of Barite. FIG. 220. The Unit Series of Pyra- mids. b^ Macro series of pyramids, & : nb : c, (hlh), when the intercept on b is greater than unity. c. Brachy series of pyramids, n& : b : c, (Ihh), when the intercept on the a axis is greater than unity. THE ORTHORHOMBIC SYSTEM 115 II. Prisms, r : b : oo c ; (hko). Prisms are parallel to the c axis; the poles of Fig. 217 will lie on the primitive circle between the digonal axes. When the pole is nearer a the form will be of the macro series, a : nb : oo c, as its in- tercept on b will be larger than unity, and when near b, the form will be of the brachy series ; Fig. 221 is the unit prism, which is the limit- ing form connecting the two series. III. Domes. Macro series of domes, na : oo b : me ; (hok). When the poles lie in the diam- etral plane containing the c and & axes, the faces will be parallel to the macro axis, and the form will be a macrodome Fig. 222, an open form bounded by four similar faces. There will be a series of macrodomes the angles of which and the inter- cepts on the axes will depend upon ... . , , FIG. 221. The Unit Prism of Barite. the position of the poles. Brachy series of domes, oo&: nb : me; (ohk). When the poles lie in the plane of symmetry containing the c -,-f 'j i FIG. 222. The Unit Macro- dome of Barite. FIG. 223. The Brachy Series of Domes. and b axes, the faces will be parallel to the brachyaxis, and the form will be a brachydome, of which there is also a series, Fig. 223. 116 MINERALOGY IV. Pinacoids. Three other positions of the poles are possible, which represent the fixed forms, that is when the poles take the position of the angles of the triangle, or coincide with the crystallograph- ical axes. Basal pinacoid, ooa: oobrc, (001). FIG. 224. The Unit Brachydome of Barite. FIG. 225. Combination of the Three Pinacoids. When the pole coincides with the c axis, the face will be parallel to a and b, the form consisting of two faces, one above and one below the equatorial plane, which will produce the basal pinacoid. Macropinacoid, & : oob : ooc , (100). Here the pole coincides with the & axis, when the face is parallel to b and c. Brachypinacoid, oo a : b : oo c, (010). Here the pole coincides with the b axis and the faces are parallel to a and c; Fig. 225 represents the three pina- coids in combination. Forms in combination. The possible forms to combine in this type therefore are : FIG. 226. Barite. Pyramids, series (hkl). Prisms, series (hko). Macrodome, series (hok). Brachydome, series (ohk). Basal pinacoid, (001). Macropinacoid, (100). Brachypinacoid, (010), Examples. A large number of, arid especially important, rock- forming minerals crystallize in this type: THE ORTHORHOMBIC SYSTEM 117 Olivine, (Mg,Fe) 2 SiO 4 . Enstatite, MgSi0 3 . Aragonite, CaCO 3 . Topaz, Al[Al(O.F 2 )]SiO 4 . Barite, BaSO 4 . Fig. 226 is a combination of the base, brachydome, macrodome, and macropinacoid in barite. CLASS, ORTHORHOMBIC HEMIMORPHIC TYPE 7, DIDIGONAL POLAR Symmetry. Crystals of this type possess one didigonal axis, the c axis, and two planes of sym- metry intersecting in the c axis, Fig. 227. It is a polar development of the didigonal equatorial, with a loss of all the symmetry lying in the equa- torial plane of that type. Forms I. Hemipyramids, u/1 na:b: me (hkl), (hkl). There would be upper and lower pyramids of each series, Fig. 228. FIG. 227. Type 7, Didigonal Polar. II. Domes. Macrodomes, u/1 - -; (hok), (hok), Fig. 229. Brachydomes, u/l( 2: b: mc ) ; (ohk), (ohk), Fig. 230. FIG. 228. The Upper Hemipyramid. FIG. 229. The Upper Macrodome. Both domes would be modified by the symmetry, yielding hemi- domes, while the prisms would suffer no apparent change. 118 MINERALOGY III. Of the three pinacoids, the base would yield hemi forms, the upper and lower base, u/l * : ' C , (001), (001). Possible forms to combine in the type would be : Pyramids upper and lower, (hkl) , (hkl) . Prisms, two series, (hko). Domes ; upper and lower macrodomes, (hok) , hok) . Domes ; upper and lower brachydomes, (ohk) , (ohk) . Macropinacoid, (100). Brachypinacoid, (010). Upper and lower base (001,) (001). Examples. Calamine, ZnSiO 3 , 2H 2 O. Fig. 231 represents a combination of two upper hemimacro- and brachydomes, the FIG. 230. The Upper Brachydome. FIG. 231. Calamine. unit prism, the lower unit hemipyramid, the macro- and brachy- pinacoids, and the upper base. Struvite, NH 4 MgP0 4 , 6 H 2 0, also crystallizes in this type. CLASS, SPHENOIDAL HEMIHEDRONS TYPE 6, DIGONAL HOLOAXIAL Symmetry. Crystals of this type possess three axes of digonal symmetry corresponding to the crystallographical axes, but no planes of symmetry. Fig. 232 illustrates the symmetry of the type. Forms I. Sphenoids, The poles of Fig. 232 represent the general form of the type. THE ORTHORHOMBIC SYSTEM 119 The right sphenoid, with four similar scalene triangular faces, Fig. 233 and Fig. 234, is the complementary left form. O FIG. 232. Type 6, Digonal Holoaxial. FIG. 233. The Right Sphenoid. Other forms are similar in appearance to those of type 8. Forms possible to combine in this type will therefore be : Right and left sphenoids, K(hkl) , K(hkl) . Orthorhombic prisms, (hko). Macropinacoids, (100). Macrodomes, (hok). Brachypinacoids, (010). Brachydomes, (ohk). Basal pinacoid, (001). Examples. Sulphur, S, Fig. 235, represents a combination of the right and left sphenoids and the base on sulphur. Epsomite, MgSO 4 . 7 H 2 O, Fig. 236, is a combination, of the prism and right sphenoid as found on crystals of this mineral. FIG. 234. The Left Sphenoid. FIG. 235. Sulphur. FIG. 236. Epsomite: Combination of (110) and (111). 120 MINERALOGY Crystalline Characters In the orthorhombic system, where the unit on each axis is a different one, there are two axial ratios, fe and - , b being the unit of comparison, or as they are generally written, a : b : c = .8152 + 1 : 1.31359 + , the axial values of barite. On calculating the axial ratios it will be necessary to measure the angle of the unit prism, or the angle be- tween the unit prism and either the macro- or brachypinacoid, when a in terms of b may be calculated. To determine c, the angle of the unit dome or the angle between the dome and a pinacoid must be measured. Example. In the mineral stauro- lite, the angle 100,110 = 25 20'; as this is the angle between the poles, the actual angle of the right-angled triangle is 25 20', with the side a opposite, therefore, tan 25 20' = = = .4734+, as b = 1. b 1 In calculating the value of c, the angle 101J01 = 110 32', which being the angle between the poles, the actual angle between the faces, Fig. 237, cao = 1/2 dod' = 55 16' ; - = tan 55 16', c = FIG. 237. tan 55 16'; a = .4734. Log tan 55 16' Log .4734 10.159083 - 10 1.675228 Log c = 9.834311 - 10 c = .6829 Axial ratio of staurolite, a : b : c = .: 1 : .6829. THE MONOCLINIC SYSTEM The monoclinic system includes all those crystals referable to three axes, two of which, c the vertical, and b the orthoaxis, are at right angles ; the third, or clinoaxis, is at right angles to b and in- clined to c, and is designated by a. Here the three diametral planes THE MONOCLINIC SYSTEM 121 no longer divide space into eight equal octants, but into octants of two different sizes, four of which are large, or obtuse, and four smaller, or acute. The two upper front and the two lower back octants are large and designated octants ; the smaller are the + octants. As the inclination of a to c varies with the substance, the angle between these two axes, measured in the + octants is designated by (3, and is therefore less than 90, which added to the two axial ratios - and - make three crystalline characters for the D D system. CLASS, HOLOHEDRAL (HOLOSYMMETRIC) TYPE 5, DIGONAL EQUATORIAL Symmetry. Crystals of this type possess one digonal axis, the b axis ; one plane of symmetry, the equatorial plane, at right angles to the c axis and containing a and c, and a center. Figure 238 represents the relation of the axes and plane of symmetry, and also the general position of _ the monoclinic crystals relation to the ob- +b in +0 FIG. 238. The Monoclinic Axes and Plane of Symmetry. server. Fig. 239 representing the symmetry of the type, it differs from others in that the b axis and not c is perpendicular to the plane of the paper. In viewing a crystal of this type held in the general position, the equatorial plane will be vertical. FIG. 239. Type 5, Digonal Equa- torial. 122 MINERALOGY Forms I. Monoclinic pyramids differ from those heretofore considered, as the octants subtended by the faces are large and small, yield- ing faces, represented by the same parameters or indices, of two sizes, of which the larger faces form the minus and the smaller form the plus pyramids. Monoclinic pyramids do not inclose space; FIG. 240. The Minus Pyramid. The Plus Pyramid. Combination of the Plus and Minus Pyramid. the combination of the plus and minus pyramids is equivalent to a single orthorhombic pyramid, and incloses space, Fig. 240. As in the orthorhombic system, there are three series of pyramids holding the same relation to the axes as in that system. _Unit series of pyramids, a : b : me, (hhl), (Mil). Ortho series of pyramids, & : nb : mC, (hkl), (hkl). - -'" ------ _Clino series of pyramids, n : b : me, (khl), (km). The unit or fundamental pyramid is the connecting link between the three series. When the form represented by the spherical projection is a pyramid, the poles will fall within the triangle, Fig. 239. . circle at right angles to the c axis, the form is a prism, of which there are two series ; when the pole is nearer the b axis, it is a prism of the clino series, ni : b : o> c, (hko). When THE MONOCLINIC SYSTEM 123 its position is nearer the extremity of the a axis, it is of the ortho series, & : nb : oo c, (kho). Prisms are not plus and minus forms, as each face subtends two octants, one above and one below, Fig. 241. III. Clinodome, oo : nb : me, (ohl). When the pole lies in the plane at right angles to the a axis, the faces are parallel to the clinoaxis and the form is the clinodome. FIG. 242. The Plus and Minus Ortho- domes. FIG. 243. Combination of the Three Pinacoids. IV. Orthodome. If the poles lie in the equatorial plane the faces will be parallel to the orthoaxis and the form is the ortho- dome, of which there are two forms: the plus orthodome, & : oob : me, (hoi), formed by the two faces subtending the four small octants ; and the minus orthodome, a : b : me, (hoi), formed by the two faces subtending the four large octants, Fig. 242. V. Orthopinacoid, a : oo b : oo c, (100), when the poles lie in the plane of sym- metry at 90 from c. VI. Clinopinacoid, oo a : b : oo c, (010), when the poles lie on the b axis. VII. Basal pinacoid, oo a : oo b : c, (001), when the poles lie on the c axis. Figure 243 is a combination of the three pinacoids. Combinations. The possible forms to combine in this type are : FIG. 244. Combination of mdlOXaClOO), b(010), u(lll), and y(101), in Augite. Pyramids, three series, plus and minus, (hkl), (hkl). Prisms, two series, (hko). 124 MINERALOGY Ortho series of domes, plus and minus, (hok), (ho.k). Clino series of domes, (ohk). Orthopinacoid, (100). Clinopinacoid, (010). Basal pinacoid, (001). Examples. Some of the most important rock-forming minerals are members of this type, as : Orthoclase, KAlSi 3 O 8 . Epidote, HCaaAlaSisOis. Augite. Gypsum, CaSO 4 , 2H 2 0. Amphibole. Fig. 244 represents a combination of the unit prism, (110), orthopinacoid, (100), clinopinacoid, (010), minus unit pyramid, (111), and the plus orthodome, (101), of augite. CLASS, MONOCLINIC HEMIMORPHIC TYPE 4, DIGONAL POLAR Symmetry. Crystals of this type possess one digonal axis of symmetry, the b axis, Fig. 245. The forms may be considered as pro- o o FIG. 245. Type 4, Digonal Polar. FIG. 246. duced by a polar development of type 5, along the orthoaxis ; thus the plane and center of symmetry is lost, and yielding hemi forms either side of the plane of symmetry each of which may occur independently, Fig. 246. THE MONOCLINIC SYSTEM Forms 125 The new forms would be : I. Right and left plus and minus hemipyramids, of two faces each, (hid), (hid), (hid), (hid), Fig. 247. FIG. 247. The Right Plus and Minus Hemipyramids. FIG. 248. Right Hemi- prism. FIG. 249. The Right Hemiclinodome. II. Right and left hemiprisms of two faces each, (hko), (hko), Fig. 248. JII. Right and left hemiclinodome of two faces each, (ohk), (ohk), Fig. 249. IV. Right and left clinopinacoid, (010), (OlO). The orthodome and the ortho and basal pinacoids are not modified, and combine with the above new forms on crystals of this type. Examples. A number of organic compounds crystallize in this type, as, FIG. 250. Milk Sugar: Com- tartaric acid, C 2 H 6 6 . bination of bno), b'(olo), Fig. 250 is a combination of the right and left clinopinacoids, right and left prism, orthopinacoid and left clinodome on milk sugar. and a (100) 126 MINERALOGY CLASS, DOMATIC HEMIHEDRONS TYPE 3, EQUATORIAL Symmetry. Crystals of this type possess a plane of symmetry only, Fig. 251. They may be considered as hemihedrons formed FIG. 251. Type 3, Equatorial. FIG. 252. by selecting pairs of faces of type 5, which meet in the equatorial plane, as the shaded faces of the minus pyramid of Fig. 252. Forms I. Upper and lower plus and minus hemipyramids of each series of two faces each, ( 1 ^Ju/1, K(hkl), K(hkl), I V 2 J K(hkf), K(hkl), Fig. 253. II. Front and | rear hemiprisms of two faces each, f / r ( n * : V C ) K(hko), K(hko). Fig. 254. III. Upper and lower clinodomes, /oo at : nb : D u/1 (- K(ohl), (ohl), Fig. 255, of two faces FIG. 253. The Lower Minus eacn - Hemipyramid. IV. Upper and lower, back and front FIG. 254. Front Hemiprism. THE MONOCLINIC SYSTEM 127 hemiorthodome of one face each, u/1 (no" : oo b : me), (hoi), (hoi), (hoi), (hoi), Fig. 256. V. The ortho and basal pinacoids, from their relation to the axis of symmetry, will each be represented by one face. I FIG. 255. The Upper Semiclinodome. FIG. 256. The Upper Front Hemi- orthodome, (hoi). The clinopinacoid will appear as in type 5 with two faces. Examples. A single mineral, clinohedrite, H 2 ZnCaSi05, crys- tallizes in this type. Fig. 257 is a combination of the plus and minus hemipyramid, (111), plus upper pyramid, (771), plus upper pyramid, (161), and the back hemiprism, (110) as found on this mineral. Crystalline Elements In addition to the two axial ratios, in the monoclinic sys- tem the angle p is required to fix the characteristics of any FlG - 257. - Combination of p(iii), q(iii), , , f rp,, . : m(110), t(771), z(161) on Clmohednte. crystal form. The angle is measured in the diametral plane, and in the plus octants; its value is therefore always given as less than 90. It may bo 128 MINERALOGY measured directly, being the angle between the base and the orthopinacoid. Four angles are sufficient to determine the elements: (100*001), (101 A 100), (001.011) and (110*100). Calculations. When p is not measured directly, generally (001 * 110) and (110*110) can be obtained, Fig. 258. In the right-angled spherical triangle, right-angled at A, the two angles B(001 A 110)andC = l/2(110 A 110) are known, the two sides c and b can be calculated. C is the angle of the right-angled triangle of which a is one leg and b the other, also p = 180 - b. Example. In orthoclase the angle (001*110) = 67 47' and the angle (110*110) = 61 13'; as these are the angles between the poles, in each case subtract- FIG. 258. triangle = 112 13' and C ing from 180, B in the spherical 1/2 (118 47'). cosB cos (112 13') C sinC "sin (59 24') Log cos 112 13' = 9.577618 Log sin 56 24' = 9.934783 Log cos b = 9.642745 b = 116 3'. P =180 - (116 3') = 63 57'. ' The axial ratio. In the spherical triangle ABC, with the side b and the angle C known, being right angled at A, the side c is calculated by Napier's rule. tan c sinb sin (116 3') cot C cot (59 23' Log sin 116 3' = 9.953475 Log cot (59 23' 30") = 9.772312 Log tan c = 10.181 163 c = 5637'. 30") THE TRICLINIC SYSTEM 129 In the triangle doe, right-angled at o and with the side oe = b = i and the angle ode known : ^P = *- = cot ode = cot 56 37' = .658 + or a = .658 + . oe b For the value of c the angle (001 A 101) = 50 16'. In the triangle coa, P = 63 37' and oca = 180 - (50 16') - (63 37') = 65 47'. In the triangle aoc, in which the angles and one side oa = a = .658 + are known, oc = c may be calculated. oc : oa : sin oac : sin oca, or c : a : : sin 59 16' : sin 65 45'. a X (sin 59 16') sin (65 450 Log a =1.818226 Log sin 50 16' = 9.885942 9.704168 Log sin 65 45' = 9.959852 Log c = 1.744316 c= .555 + . The crystalline constants of orthoclase would be expressed as calculated, a : b: c = ,658+ : 1 : .555+ : p = 63 57'. THE TRICLINIC SYSTEM In the triclinic system all axes are inclined, and none of the five crystalline elements are fixed ; the axes are unequal and designated, &: b: c, as in the orthorhombic system. Generally the unit plane has been chosen so that the unit on c is smaller than that on b, but this may not be so in all species. Here the diametral planes divide space into octants of four different sizes ; of which opposite octants through the center are similar; thus the pyramids of the triclinic system will consist, at the most, of a single pair of parallel faces each subtending octants of the same size. The four possible pyramids are equivalent to the orthorhombic pyramid and in combination inclose space. The axial angles are either greater or less than 90 and are measured in the plus octant, the upper right- hand octant. The angle between b and c is designated a, that between a and b, -y, and that between & and c, P, Fig. 265. 130 MINERALOGY CLASS, HOLOHEDRAL (HOLOSYMMETRIC) TYPE 2, CENTROSYMMETRIC All forms of this type possess a center of symmetry only and each form is composed of a single pair of parallel faces, Fig. 259. Forms. Pyramids. FIG. 259. Type 2, Centrosym- metric. FIG. 260. Two Faces forming a Triclinic Pyramid. Unit series of pyramids, |a:b:mc, (hkl). a:b:mc, (hkl). a:b:mc, (hkl). a:b:mc, (hkl). FIG. 261. Two Faces forming a Triclinic Prism. THE TRICLINIC SYSTEM 131 Macro series of pyramids, a:nb:mc, (hkl). a:nb:mc, (hkl). arnbrmc, (hkl). a:nb:mc, (hkl). FIG. 262. The Two Faces which form a Triclinic FIG. 263. Combination of the Dome. Three Pinacoids. Brachy series of pyramids, na : b : me, (hkl) narbrmc, (hkl) na:b:mc, (hkl) na:b:mc, (hkl). Fig. 260. FIG. 264. Axinite: Combination of m(110) (201), FIG. 265. Combination of the Three Pinacoids in Rhodonite. Macro series of prisms, a : nb : oo c, (hko) . a : nb : ooc, (hko). 132 MINERALOGY fn : b : ooc, (hko). ^. OC1 Brachy series of prams, | na . b . , C) (h i o) . F 'g- 261 Macro series of domes, Brachy series of domes, na : oo b : me, (hoi) . na : oo b : me, (hoi) . f oo a : nb : me, (ohl). I oo a : nb : me, (ohl). Fig. 262. , Macropinacoid, a : oo bj^ oo c, (100) . Brachypinacoid, oo a : b_: oo c, (010). Basal pinacoid, ooa: oob : c, (001), Fig. 263. Examples. The plagioclase feldspars crystallize in this type. Fig. 264 represents a combination ofJUO) (110) (100) (111) (201) on a crystal of axinite, in which a : b : c = .492 + : 1 : .479 + and a = 82 54', p = 91, y = 131 32', 100 A 010 = 48 21'. Fig. 265 is the combination of the three pinacoids in rhodonite, MnSi0 3 , in which a = 103 18', p = 108 44', ? = 81 39'. CLASS, TRICLINIC HEMIMORPHIC TYPE 1, ASYMMETRIC Symmetry. Crystals of this type possess no symmetry what- ever, and each form of the type is composed of a single face, Fig. 266. Any collection of faces, how- ever irregularly grouped, may belong to this type, provided they conform to the law of ra- tional indices. All forms of the type may be considered as being produced by the selection of one face of the pair form- _ FIG. 26/. Calcium Thio- ing the corresponding holohedron of type 2. Forms, Possible forms will be the same as in type 2, except they will all be hemi forms consisting of one face each, and designated as the upper right front hemipyramid, or as the lower back hemimacrodome, etc. FIG. 266. Type 1, Asymmetric sulphate : Combinations ofc(001),b(010),m(110), q(011), andh(HO). THE TRICLINIC SYSTEM 133 Examples. There are no minerals crystallizing in the type, but several salts, as strontium bitartrate, Sr(C 4 H 4 06)2 . 5 H 2 O. Calcium thiosulphate, CaS 2 O 3 . 6 H 2 O, Fig. 267, represents the combination of (001) (010) (HO) (Oil) (110) common on this salt. The Crystalline Elements of the Triclinic System In the triclinic system, where no elements are fixed, all five are calculated from measured angles, as none of the axial angles can be measured directly; at least five angles must be measured. The angles generally chosen are the pinacoidal angles, 001 A 100 ; 001 A 010; 100 A 010, and the angles between the unit form and a pinacoid, as 100 A 1 10 ; 001 A 101 ; 001 A 01 1 . When any five of these angles are measured, the axial ratios and axial angles may be calculated. CHAPTER VII RELATION OF INDIVIDUAL CRYSTALS CRYSTALS as found in nature are rarely simple, or composed of one individual. During the process of formation they must necessarily come hi contact with each other ; this contact modifies them, not only producing distortions and irregularities in external form, but reentrant angles are formed. The angles of all simple crystals must be less than 180, and whenever an angle greater than 180 or a reentrant angle appears, it is proof that the crystal is of a compound nature or consists of more than one individual. At the time of separation, one crystal may have an influence upon the position or direction of the axes of its neighbor, and this influence may show itself in various ways. Minerals totally different in composition and crystalline structure are sometimes so placed that certain axes and edges are parallel in the two species, as in case of staurolite and cyanite from St. Gothard, Fig. 268 ; while belonging to different crystal systems, these two v^S $ ^WHH species are often *jj#j- v \ ^ r so placed that |t, ;fek|. 40Q "'* their c axes are ^It -i^-if W^ $ parallel. Such v* ""^~W ''* parallel growths, however, occur the more often be- tween individuals of the same species, or between species belonging to the same isomorphic group; in such cases large aggregates will have all their crystalline directions parallel as in Fig. 269, sulphur, or as in Fig. 270, microcline from Pike's Peak. In such parallel growths equivalent faces will reflect light or appear bright at the same time. 134 FIG. 268. Cyanite and Staurolite in Parallel Position from St. Gothard, Switzerland. RELATION OF INDIVIDUAL CRYSTALS 135 It often happens that the faces of a large crystal may have a mat- like surface, caused by a layer of small individuals which have been deposited, generally in parallel position, upon the surface of the larger crystal. They either repre- sent a secondary generation, or are the result of changed condition during crystalliza- tion, causing a more rapid deposition, Fig. 271. Drusiness of faces is also produced either by a second generation of the same species, or by secondary minerals formed by the decomposition of the surface of the mineral upon which the small crystals are placed. Complete par- allelism exists the more often be- tween individ- uals of the same species, or the species of an iso- morphous group. The dividing line between individ- uals is not al- ways distinct, for as each individ- ual is reduced in FIG. 269. Parallel Aggregate of Sulphur Crystals. Girgenti, Sicily. FIG. 270. Parallel Aggregate of Microcline from Pike's Peak, Colorado. size, each may vanish as a line, 136 MINERALOGY and will be repre- sented by a striation running across the crystal face in a fixed direction. Wherever these striations appear on the face of a crys- tal, they must be considered as the boundary between two individuals. In Fig. 272 a, a com- plex quartz crystal, the individuals are well marked and apparent ; but in Fig. 272 b, a quartz crystal with striations running across the prism face parallel to the intersection of the prism and rhombohedron, each striation represents a reentrant angle between individuals, or the crystal in its growth may be said to oscillate between the FIG. 271. Quartz Crystals in Parallel Position on Or- t thoclase. b a FIG. 272. Smoky Quartz from Disentis, Switzerland. RELATION OF INDIVIDUAL CRYSTALS 137 prism and the rhombohedron. Striations are very characteristic of certain crystal faces in various mineral species. The cube face FIG. 273. Crystals of Pyrite showing Striations on the Cube and Pyritohedron. in pyrite is striated at right angles to a pair of edges, Fig. 273, representing an oscillatory growth between the cube and the pen- tagonal dodecahedron or py- ritohedron. In tourmaline, Fig. 274, the prism faces are striated lengthwise the crys- tal, which represents an os- cillation between the trigonal and hexagonal prisms. Twins. In a large num- ber of unions of crystals, all crystallographic equivalent directions are not parallel, as in parallel growths; some may be parallel and others at an angle, as if rotated around an axis 180, or as if reflected across a plane. Fig. 275 is a diagrammatical representation of 6 molecules. In a, b, and c the equivalent directions are all parallel, as in a simple crystal, but x, y, and z are reversed, as if reflected over the plane FIG. 274. Tourmaline from Pala, Cali- fornia, showing Longitudinal Striations on the Prism. 138 MINERALOGY mn. Again, a, b, and c, when revolved around mn as an axis 180, will become congruent with x, y, and z. The molecules a, b, c are said to occupy a twinning position in regard to x, y, z/and the two individuals are said to be twins. The axis of revolution is the twinning axis, and a plane at right angles to the twinning axis is the twin- ning plane. The plane m ! _ ! ' n separating the two indi- viduals is the composition or contact plane ; this with rare exceptions is parallel to a possible crystal face. The twinning axis is either parallel to a possible crystal edge or perpendicular to a possible face. It can never be an axis of even symmetry, as by a revolution of 180 around such an axis the two individuals would be congruent and form a simple crystal. Fig. 276 represents a simple crystal of gypsum ; Fig. 277 is a twin FIG. 276. Gypsum Crystal showing the Position of the Twinning Plane. crystal of gypsum in which the twinning axis is parallel to the vertical axis c. Fig. 278 is a tw'nned crystal of gypsum in which it may be seen that one individual has been revolved around the RELATION OF INDIVIDUAL CRYSTALS 139 twinning axis c 180, and also that the twinning axis is parallel to the edge in the prism zone. The shaded plane is the contact plane and is parallel to the or- thopinacoid. The trace of the twinning plane in the crystal is usually marked by a reentrant angle, as xyz, or, where this is reduced to a minimum, by stri- ations on the crystal face, as yy', Fig. 278. A reentrant angle is not al- ways present to distinguish the crystal as a twin, and often when absent, as is shown in the epi- dote crystals from Prince of Wales Island, Alaska, Fig. 280, where the twinning axis is per- FlG . 277. - Gypsum Twins from near pendicular to the orthopinacoid Paris, France. and the composition and twin- ning plane is the orthopinacoid, and which after a revolution of 180, leaves on these crystals no indication of the twinning. Striations on the clino- pinacoid due to parallel growth are indicated by the parallel lines, and the effect of twinning on these striations is shown. The striations meet the twin- ning plane, yy', from each individual at the same in- clination, and the trace of the twinning plane on the crystal face bisects the angle between them. The complexity of some apparently simple crystals FIG. 278. Gypsum Twins. J , is often only revealed by the microscope and polarized light, as in the twinning bands of the plagioclase feldspars, Fig. 281. In enantiomorphic types, 140 MINERALOGY where there are right- -and left-handed forms, it is not possible to revolve one individual around an axis into a congruent position. FIG. 279. Simple Crystal of Epidote. FIG. 280. Twinned Epidote from Prince of Wales Island, Alaska. and a twinning axis is therefore not always possible; but such crystals are twinned by reflection, as some twins of quartz. Twins formed by the union of plus and minus, upper and lower, or right FIG. 281. -Twinning Lamellae of Plagioclase, between Crossed Nicols. Much enlarged. and left forms are supplementary twins; and when as contact twins, and equally developed, the individuals will possess a pseudo- RELATION OF INDIVIDUAL CRYSTALS 141 FIG. 282. Supplementary Twins of Pyrite. symmetry or a symmetry of a higher type, as the upper and lower forms of a polar type when joined along the plane of the base will possess the symmetry of an equatorial type. The plus and minus forms may penetrate each other and be distinguished as complex individuals by the reentrant angles. Fig. 282 is a drawing of the supplementary twins of pyrite, formed by the plus and minus pyritohedrons ; while Fig. 283 is a photograph of these twins from Prussia. In interpenetrating twins there is no marked plane of contact between individuals, but a very irregular and ill-defined area separates the two indi- viduals internally. In the growth of crystals the twinning position may have been assumed at the very outset, in the nucleus of crystalliza- tion, and the complex nature existed at the beginning of crystallization; or again the individuals may have developed as simple crystals* when through a changed condition mole- cules have separated in a twinned position and the axis of the simple crystal is abruptly changed. Where there is but one angle in the axis of an elongated crystal they are often termed genic- ulate twins, as the geniculate twins of rutile, Fig. 284. This bending or angle in the axis of the crystal may be repeated either in the FlG . 284 ._ Geniculate Twins of Rutile from Same direction Or in the Op- Lancaster County, Pennsylvania. FIG. 283. Supplemen- tary Twins of Pyrite. 142 MINERALOGY FIG. 285. Cyclic Twins of Marcasite from Folkstone, England. posite direction. When repeated in the same direction a number of times, the complex individual is circular and is termed cyclic twins, as in marcasite, Fig. 285 ; or the twinning may be repeated first in one di- rection and then in the other, with a zigzag effect, as in rutile, Fig. 286. When the twinning is repeated many times at very short intervals, each individual of the complex structure will be confined to a very thin sheet pass- ing through the aggregate, parallel to the composition plane, and indicated on the crystal externally by a re- entrant angle as illustrated in Fig. 287, a twin crystal of albite in which the clinopinacoid is the composition plane and the twinning axis is perpendicular to this face. Fig. 288 is a crystal of albite composed of several individuals twinned after this same law ; each individual is indicated by the reentrant angle passing around the crystal parallel to the twin- ning plane. Each individual may be reduced to extreme thinness, when only a striation on the crystal face will remain to mark the plane of contact separating individuals, the whole complex structure building up an appar- ently simple crystal. When often repeated in this manner, the twinning is termed polysynthetic. * IG ' 286> ~ zigza g- twins of Rutile - The striations produced by polysynthetic twinning are quite dif- ferent .from those caused by parallel growths; the former pass through the body of the crystal and are caused by the arrange- ment of the molecules and will therefore appear, not only on crystal faces, but also on all cleavage faces intersecting the twinning planes, RELATION OF INDIVIDUAL CRYSTALS 143 as in the plagioclases. The latter are confined to the crystal face and are not caused by a change in the molecular arrangement, and will therefore not appear on cleavage faces or be indicated below the surface. Twinning in the isometric system. In isometric minerals the spinel law is the most common. In this method of twinning the FIG. 287. Twins of Albite. FIG. 288. FIG. 289. Spinel Twins. trigonal axis, which is common to the five types of the system, is the twinning axis, and the face of the octahedron 111 or the plane parallel to it is the com- position plane. For all minerals crystallizing in the isometric system this is a possible form of twinning. Fig. 289 is a drawing in which mn is the twinning axis, and Fig. 290 is a photograph of spinel twins from Frank- lin, New Jersey. Penetration twins with the same axis as the twin- ning axis occur in galena and fluorite, Fig. 291, twins of fluorite. The second law is where the twinning axis is per- pendicular to the face of the rhombic dodecahedron face, 110, and is a possible twinning law in those types only where this axis is FIG. 290. Spinel Twins from Franklin, New Jersey. 144 MINERALOGY FIG 291. Penetration Twins Fluorite. not an axis of symmetry, as the tesseral central and tesseral polar types. The supplementary twins of pyrite are of this class. Twinning in the tetragonal sys- tem. The general law hi this system is where the twinning axis is perpendicular to a pyramid face and is a possible mode of twinning in all seven types of the system. Fig. 292 is a drawing of twins of cassiterite, in which the twinning axis is perpendicular to the pyr- amid face of the sec- ond order, 101, and the composition plane cc is parallel to 101. . Fig. 293 is a photograph of a nat- ural twin from Zinnwald, Bohemia. In the ditetragona) polar type, the normal to the sphenoids 1 1 1 is the twinning axis, with the sphenoidal face the composition face ; when developed as contact twins, they are similar to the spinel twins of the isometric system. In the tetragonal polar types a third twinning law is possible, as in these types the lateral axes are not axes of symmetry and are therefore possible twinning axes. These a x re supplementary twins, and when there is no reentrant angle at the equatorial plane there is nothing to indicate the twinned nature of the crystal, and the symmetry is apparently that of an equatorial type. Most crystals of wulfenite are of this character. Twinning in the hexagonal system. Twins in the hexago- nal division are rare. A pos- sible law in all the types of the system is where the twinning FIQ. 292. Twins of Cas- siterite. FIG. 293. - Cassiterite Twins from Ziinn- wald, Bohemia. RELATION OF INDIVIDUAL CRYSTALS 145 axis is perpendicular to a pyramid face and the pyramid face is the composition plane. This law is a very common one in the trigonal types, and is the same as where the twinning axis is perpendicular to a scalenohedral or rhombohedral face. In calcite the common form of twinning is where the twinning axis is normal to'the rhombohedron e (Oil 2), Fig. 294. FIG. 294. Calcite Twins in which e (0112) is the Composition Face. Guanajuato, Mexico. In types of alternating symmetry and trigonal types the vertical axis is a trigonal axis, and is therefore a possible twinning axis with the base as the composition plane ; Fig. 295 is such a twin of cal- cite. In the polar types supplementary twinning, as in the tetrag- onal system, is a possible law. There are very few minerals of these types; the most common is tourmaline, in which there are no twins ; and in nephelite the polar symmetry is shown by the etch figures only, and all crystals must be considered as examples of supplementary twinning. In the holoaxial types, of which quartz is an example, twinning by reflection is the rule, as in the Brazilian twins, where the plane of reflection is parallel to the prism face (1120), Fig. 296, and x is a reflection of x'. 146 MINERALOGY FIG. 295. Twins of Calcite in which the Twinning Axisis c and the Composition Plane is the Base. Guanajuato, Mexico. Twinning in the orthorhombic system. There are three classes of twins possible in all three types of the system : I. Where the twinning axis is normal to a pyramid face. This is in fact a pos- sible law in all 32 types of crystals, consid- ering the octahedron and its resulting hemihedral forms as pyramids. In Fig. 297 a, twins of staurolite, in which the twinning axis is normal to the pyramid (232). II. Where the twinning axis is normal to a prism face. Fig. 298 is a diagram of aragonite, in which the twinning axis is normal to the prism 110, often repeated, and as the prism angle is nearly 120, the FIG. 296. -Brazilian Twins twinned crystals have a pseudohexagonal of Quartz. symmetry, Fig. 299. RELATION OF INDIVIDUAL CRYSTALS 147 III. Where the twinning axis is normal to a dome, as is well illustrated by the cross-shaped twins of staurolite, Fig. 297 b, in which the twinning axis is normal to the dome (032). a b FIG. 297. Staurolite Twins from Georgia. In the didigonal equatorial type all faces may be twinning faces, except the three pinacoids, the normals to which are the axes of FIG. 298. Twins of Aragonite, Twin- ning Plane 110. FIG. 299. Triplets of Aragonite from Bastanes, France. symmetry. In the sphenoidal and digonal holoaxial types, these normals are still digonal axes of symmetry, and only the above three types are possible. In the digonal polar type supplementary 148 MINERALOGY twins are possible, as here the lateral axes are not digonal axes of symmetry and may be twinning axes with the base as the com- position plane. These supplementary twins are well illustrated by calamine. Twinning in the monoclinic system. The only direction not possible as an axis of twinning is the orthoaxis, and twins are very common in the system. They may be divided into the three types as in the orthorhombic system, with the addition that the two pinacoids may also be twinning planes. In gypsum and augite the twinning plane and composi- tion plane is the orthopinacoid, and the twinning axis is the normal to it. In the Carlsbad twins of orthoclase the twinning axis is the vertical axis, while the composition plane is the clino- pinacoid, parallel to it, Fig. 300. Twinning in the triclinic system. As there is only a center of symmetry, any plane is possible as a twinning plane and twins are FIG. 299 a. Diagram of the Basal Plane of Aragonite Triplets, showing the Rela- tion of the Individuals. FIG. 300. Carlsbad Twins of Orthoclase. Brice, New Mexico. numerous and complicated, often repeated polysynthetically as in the plagioclase feldspars. CHAPTER VIII ON THE MEASUREMENT OF CRYSTALS AND THE USE OF THE GONIOMETER IT is often necessary to measure the angles between two crystal faces in order to identify the forms present on the crystal, partic- ularly when the specimen is much distorted. This may be true even with such well-developed and easily recognized forms as the prism and rhombohedron on quartz. The forms present in com- plicated combinations must always be proven by the measurement of the angles between the faces. The measurement of the angles will also help in the identification of the mineral species ; and in chemical compounds their variation from the theoretical value may afford a means of estimating the purity of the compound, as chemi- cally pure substances possess a constant and characteristic angle be- tween crystal forms, though in isomorphous groups these angles are very nearly equal ; yet when pure each member of the group will possess an angle distinctly its own. The goniometer and the principles of the reflecting goniometer have been described in Chapter I. . For the identification of crystal forms when the crystals are not too small, the Penfield card contact goniometer, model B, Fig. 11, is a very convenient and sufficiently accurate little instru- ment, and it has the advantage of cheapness, so that each student may be provided with or possess one. It answers also as a protrac- tor and scale in the drawing of crystals. In using the instrument, the card with the scale is held at right angles to the edge and one of the crystal faces, the angle between which and the adjacent face it is wished to measure. The arm of the instrument is then rotated until it is in contact with the second crystal face ; the crystal with the goniometer in place is now held up to the light, with the line of vision parallel to the edge between the two faces ; the arm and card are 7 carefully adjusted to fit the two faces and the angle between them, so that no light is seen be- tween the instrument and the crystal faces. The instrument must 149 150 MINERALOGY be held perpendicular to the crystal faces, as the true angle is obtained only in this position; after satisfactory adjustment the angle is read from the card. This model also has the advantage of giving both the actual angle between the faces and the supplement to it or that between the poles, which is the angle usually recorded in the description of crystals. With smooth faces and as large as a centimeter across, the angles between them may be measured with the contact goniometer to within one degree, an accuracy sufficient for the identification of forms and species. Fig. 301 is a more expensive instrument, in which the two arms are detached from the scale and one moves along the other, which enables one to measure crystals separated by reentrant angle. To gain experience in the use of the instrument it is well for the student to measure the angles and identify the faces on a distorted Her- kimer County quartz crys- tal of about 1.5 cm. in diameter. When more accurate FIG. 301. -Contact Goniometer. WOI>k ls re( l uire( l, as in the calculations of the crystalline constants or characters of any mineral and in the determination of the indices of the faces as well as the identifica- tion of new forms, the reflecting goniometer is used. There are several varieties of this instrument. One, the single- circle goniometer, in which the angle is measured between the poles of the two faces in question. Another, the two-circle goniometer, by which the pole of any face is located, in reference to some chosen face as the base. The face may be said to be located by its lati- tude being measured on one circle and its longitude being meas- ured on the second circle at 90 to the first, just as a point on the earth's surface is fixed. There is also a more complicated instru- ment in which three graduated circles are used. Of these instru- ments only the single-circle goniometer will be described. The card contact goniometer may be easily converted into a single- to reflecting goniometer, for use in measuring crystals too small to yield results sufficiently accurate by the contact method of measurement. If a bridge be cemented on the arm over the eye- THE MEASUREMENT OF CRYSTALS 151 let or axis of the instrument, the crystal to be measured may be fastened on this with wax and measured.. The small crystal is then mounted with the edge to be measured perpendicular to the card, and the edge should coincide as nearly as possible with the axis of the arm ; then with the edge of the card placed on the edge of the table and with the eye at a distance of a foot and a half, the arm is revolved until one of the faces reflects the light, when a reading is taken. The instrument is replaced with the light, card, and eye in the same relative positions, which may be as- sured by getting the reflection on the same face without moving the arm ; the arm is now revolved until the second face reflects the light, when another reading is taken. The difference be- tween these two readings will be the angle between the poles of the two faces reflecting the light. Results obtained by this method are more accurate than those obtained by the contact method. It may be unnecessary to state that the accuracy of the measure- ments is increased with the distance of the light and the eye from the crystal. The principles of the reflecting goniometer have been sufficiently illustrated by the measurement of a crystal with the card con- FIG. 302. The Fuess Single-Circle Reflecting Goniometer. One Quarter Natural Size. 152 MINERALOGY tact goniometer as just described above. The accuracy of the re- sults obtained will depend upon the size of the source of light used; on the exact parallelism of the intersecting edge between the two faces with the axis of the instrument ; and upon the plane of reflec- tion, the plane in which the angle is measured being at exactly 90 to the edge of the crystal and the axis of the instrument. The instrument is constructed with all of these conditions in view and is provided with devices allowing of adjustments to these ends. Fig. 302 is the usual form of the Fuess single-circle goniometer one- fourth natural size. The collimator Z is supported by the post A, whi ch is rigidly fixed to the frame of the instrument. The collimator is provided with a lens at the inner end, and at the focus of this lens at the outer end the slit admitting light is placed. The shape and size of this opening may be adjusted in the more expensive instru- ments to suit the work at hand. The usual form is that illustrated in Fig. 303, and known as the Websky signal slit, hourglass in shape, with its vertical plane of symmetry vv' parallel to the axis of the instrument and fixed in this position. The telescope B is supported by a similar post but attached to the disk or circle upon which the vernier of the scale is marked, and can be revolved about the axis of the instrument, and rigidly fixed in any required position by the set screw C. The telescope is fitted with an eyepiece Q, which js provided with cross hairs at right angles, one of which is fixed parallel to the axis of the instrument, the other will be at 90 to the axis. The eyepiece is adjusted to this position by the collar which clamps on the eyepiece and which fits in a notch in the drawtube of the telescope, thus always assur- ing the correct position of the vertical hair when the eyepiece is withdrawn and re- turned to its position. The two cross hairs should divide the Websky signal orthorhombically when the telescope is set directly opposite the collima- tor ' as illustrated in Fig. 304. The telescope eyepiece is adjusted to parallel rays; and when it is wished to view the crystal being measured, the lens D is placed before the tube and focuses the rays on the axis of the instrument. This lens may be revolved out of the field, when the signal will appear if a face is in position. The circular disk shown under d is the graduated circle, which is THE MEASUREMENT OF CRYSTALS 153 divided to half degrees and with the vernier may be read to min- utes. This circle is revolved by the pilot wheel f, which in turn may be clamped to the axis of the instrument by the set-screw b. This circle may be accurately turned to any particular point by first revolving it with the pilot wheel and then setting the screw a and using the fine adjustment, or tan- gent screw F. The crystal to be measured is cemented with wax to a carrier which fits in a socket in the table of the instrument and is held in place by the screw p ; the screw d allows this table to be elevated or lowered until the edge to be meas- ured may be seen in the telescope. In order to adjust the crystal there are four movements neces- FIG 304 sary, each of which is controlled by a separate screw : two, r and q, are screws at right angles to each other and allow the carrier to be pushed back and for- ward ; and the two screws n and o are connected with sections of cylinder the axes of which are at 90 ; these allow the crystal to be tilted in planes at 90. By these four screws the edge to be measured may be quickly brought to coincide with the axis of the goniometer and therefore at right angles to the plane of reflection, and the edge after adjustment will also be in focus when the lens D is before the telescope. Measurement of a crystal. Let the crystal selected for measure- ment be one of topaz from Thomas Mountains, Utah, as these crystals are combinations of forms of several zones, and usually they possess bright smooth faces. The crystal is first cleansed with alcohol and ether and then not touched with the fingers to dull the faces and spoil their reflecting qualities. The various zones pres- ent are noted and rough sketches or sections at right angles to each zone made; each face in the zone sketched is represented by a letter on the drawing and the letter placed opposite the reading taken of the face when the angles are measured ; this enables each reading of the goniometer to be referred to the right face. The crystal is now mounted on a carrier with the edges of the zone to be first measured perpendicular to its surface ; the carrier 154 MINERALOGY* is now clamped in the instrument. Let the crystal be first mounted on the flat basal cleavage and the prism zone the first zone to be measured, the edges of which are all at right angles to the basal cleavage. The crystal is elevated or depressed by means of the screw d until it is in the plane of reflection. With the screw b set and the screw a loose, the telescope is revolved until its axis is at about 110 with the collimator and there set with the screw C. The lens D is placed before the telescope and the goniometer light before the Websky slit in the collimator; the crystal is now adjusted. With the right hand on the screw q and the axis of this screw at 90 to the telescope, and the left hand on the screw n, the crystal is pushed back and forth with the right hand until an edge is seen in the telescope, when with the left hand this edge is tilted until it is parallel to the vertical hair, and it is placed directly on the hair with the right hand. The crystal is now revolved with the pilot wheel f 90 to the right and the right hand is placed on the screw r and the left on the screw o ; the same edge is adjusted to coincide with the vertical hair as before. After these adjustments have been carried out accurately, the crystal when revolved will turn on the edge as an axis. This edge will now lie in the axis of the instrument and will therefore be at 90 to the plane of reflection and in position to measure. The crystal is now rotated until one of the adjacent faces to the edge adjusted is seen in the telescope to reflect the light from the collimator, the lens D is lifted and the signal will appear. The sig- nal is revolved by means of the pilot wheel f to the vertical hair ; with the screw a set, the hair is made to exactly divide it, by means of the tangent screw f ; if the adjustments have been accurate, the two hairs will divide the signal as illustrated in Fig. 304. If they do not, they are made to do so by slightly readjusting the crystal. The crystal is now revolved until the adjacent face reflects the signal ; and as this is in the same zone, it will need but little ad- justment to bring the signal into symmetrical position. Before taking any readings both signals should come into position with a simple revolution of the crystal and without any adjustment of the screws connected with the crystal ; when this is the case, the crys- tal is in position to measure. It is always best to take the first reading, in the measurement of a zone, near the zero on the circle, but on the 359 side and to revolve the crystal so as to decrease the number of degrees in each succeeding reading; they will then all stand in the column, so THE MEASUREMENT OF CRYSTALS 155 that any one below may be subtracted directly from any one above it. In the prism zone on the crystals of topaz from Thomas Moun- tains there are usually two prisms and the brachypinacoid, yield- ing ten readings to complete the 360. Fig. 305 is a plan of this zone with the faces lettered according to the usual practice. Let the first reading be taken of the face m. With the edge m A 1 ad- justed the signal from m is brought to the vertical hair as described, then a reading of the vernier is taken with a lens, and the number of degrees and minutes recorded in the notes opposite the letter standing for the face on the sketch. All faces of the zone are measured in the same manner, and as their inter- secting edges are parallel, but little adjustment should be necessary as each edge in order is brought to the ver- tical hair. It is usual in m> accurate work to make three readings of the same angle, using different parts of the graduated circle each time, to avoid being influenced by the same number ; and the average of these three results is taken as being more nearly correct than any one. It is also well to note opposite each reading the character of the signal reflected by the face, as to whether it is well defined and bright, or irregular, .dull, diffused, or complex from striations, as well-defined and bright signals will yield results nearer the truth than any poor signal will and all readings from bright sharp signals are to be given more weight in results. The following are the results of the readings in the prism zone of such a crystal of topaz : FIG. 305. Section of a Topaz Crystal 90 to the Prism Zone. m 1 b I' READINGS = 358 42' = 339 55' = ^96 33' = 253 04 m' = 234 24' " = 178 42' = 159 52' ANGLES b A m =629';b A l = 4322' b A m' =629';b A l' = 4329' 1 A 1' =86 51' m A m' =124 18' m' A m" =55 42' l' A l" =93 12' 156 MINERALOGY READINGS ANGLES b' A m" =62 8'; b' = 1163' b'.m'" = 62 8' ; bM 1'" = 73 05' I'M'" = 86 47' m'" = 54 26' m'" A m" = 124 16' m' = 358 43' m"' A m = 55 43' 43 18' '" 43 29' The signals yielded by the prism 1 are complex from striations and therefore the angles vary considerably. From the above measurements the angles for the two prisms are: for m, m" A m'" + m'" A m' = 111 25' ^ 2 = 55 42.5'; and for 1, 1*1' + 1" A 1'" = 173 38'-:- 2 = 86 49'. As the prism m has been selected as the unit prism, it will intersect the macro- and brachy-axes at unit lengths, or these lengths will be in the ratio of the unit on the b axis to the unit on the a axis. In order to determine this ratio with sufficient accuracy for use in the draw- ing of the crystal, lay off, Fig. 306, ob, equal to unity on the ma- croaxis, say 5 cm., and draw oa, the brachyaxis, at 90, then draw om, making the angle aom = 1/2 (55 42.5') = 27 51' ; from b draw ba perpendicular to om and where it cuts the a axis will be unit length from o, as oa = unity on &, which by measurement = .52 + , or oa = 52/100 of ob. Having the units on the axes a and b the parameters and indices of 1 may now be determined; in the same way from o draw ol, making the angle bol = 1/2 (86 49') = 43 25, and from b draw bl at right angles to ol, and where it cuts the a axis at x is its intercept when it cuts the b axis at unit length, ox is by measurement just twice oa; the parameters * of 1 will be, therefore, 2a : b: ooc and its indices (120). m = a:b:ooc, The faces b, b', since their normals bisect the angles of these two prisms, is a pinacoid, and its indices and parameters may be writ- ten at once as oo a: b: oo c, (010), the brachypinacoid. FIG. 306. THE MEASUREMENT OF CRYSTALS 157 Measurement of the pyramid zone. The crystal is now removed from the holder and remounted with one of the edges of the pyra- mid zone perpendicular to the flat surface of the holder. It is then clamped in the goniometer and an edge adjusted as before; the edge first selected for measurement should be that between the unit prism and the first pyramid, as all the pyramids are in the zone with the unit prism. The first reading is taken from m and con- tinued around the zone, taking the faces in order through the 180 until m" is reached. The following are the results of measure- ments in this pyramid zone: FACES READINGS ANGLES m = 185 22' c A m = 90 01' o = 159 15' C A = 63 54' u = 140 56' C A U = 45 35' i = 129 36 c A i = 34 15' c = 95 21' i" = 61 07' C A i" = 34 14' u" = 49 44' C A U" = 45 37' o" = 31 26' C A O" = 63 55' m" = 5 23' c A m" = 89 58' It is seen that the face c is 90 from the prism and is therefore the base; its parameters and indices are oo &: oo b: c, (001). The second pyramid zone, including the prism faces m'", m", may be measured and averaged with the readings of the first. From the above results the indices of the pyramids present are determined graphi- cally as follows. Some one of the pyramids must be chosen as the unit pyramid or the fundamental form in order to arrive at the unit on the axis c ; for this pyramid u has been chosen. To determine the length of c, draw, Fig. 307, oc the vertical axis and om at right angles to it, making om = om of Fig. 306, as om is the trace of the zonal plane, at right angles to the prism face. It is the plane in which all the poles of the pyramids lie and is therefore the plane in which all the angles have- been measured; then in Fig. 307, from 'm draw mi, making m FIG. 307. 158 MINERALOGY the angle omi = c A i and omu = c A u and omc = c A o ; where these lines, mi, mu, me, intersect the axis c will be their intercepts on c when it is unity on b. As u is the unit pyramid, ou will be unity on c, and by measurement and comparison to the unit on b = .47 + . The axial ratios as determined are, &:b:c = .52 + : 1 : .47 + ; as cal- culated they are, d : b : c = .5285 : 1 : .4769. In comparison, the intercept of the pyramid o on the axis c, oc, is just twice ou, and the intercept of i, oi, is 2/3 of ou. The parameters and indices of the three pyramids may now be written as follows : u = a:b:6, (111). = a:b:2C, (221). 1 = a:b:2/ 3 c, (223). There are usually two brachydomes present, and these may be measured next by remounting the crystal with an edge of this zone perpendicular to the holder and adjusting an edge in the goniom- eter as before. Starting with the first reading from one of the faces further from the base the results obtained are as follows : FACE READINGS = 179 20' = 160 42' = 117 1' = 73 23' = 55 41' cy = 62' 20 ANGLES c A y = 62 19' c A f = 43 41' c A f ' = 43 38' c A y' =62 21' c A f =43 39' FIG. 308. The parameters and indices of these two forms are determined as follows : as they are in the brachypinacoidal zone they will be parallel to the a axis. In Fig. 308, make ob equal to unity on the macroaxis and draw the vertical axis at o, then draw bf making the angle obf = 43 39' ; if oc is unity on c, then of is twice oc or 2 c and the parameters and indices of the form f are oo a b 2 c, (021). In the same way draw by, . making the angle oby = 62 20', then oy is 4 c and the THE MEASUREMENT OF CRYSTALS 159 parameters of y are oo a : b:4c and its indices are (041). The parameters and indices of any other form occurring on the crystal may be determined in a similar way, and with the data obtained the crystal may be drawn, by consulting the chapter on the drawing of crystals. CHAPTER IX OPTICAL PROPERTIES OF CRYSTALS IT has been shown that crystal forms are dependent upon and are the result of a definite arrangement of the molecules. In some cases substances which differ chemically may crystallize with al- most the same angles and forms. Again, substances which, upon chemical analysis, as pyroxene and amphibole, may yield the same percentage result, crystallize with angles which are different. Such substances may be easily identified when comparatively large specimens and well-developed forms are at hand ; but when in small fragments, even chemical analysis will fail, and yet each fragment will possess the peculiar molecular arrangement in which the one species will differ from the other, and in this case pyroxene from amphibole. It is well known that light in its passage through any medium is modified in its velocity, direction, and vibrations. These various modifications of transmitted light are the effect, in part, of the molecular arrangement, and these effects are constant and char- acteristic. They are therefore reliable when used in the identifi- cation of crystalline compounds, and just as much so as chemical tests, while in many cases they are much less troublesome in their application. According to the accepted theory, light is propagated in a medium which heretofore has been purely imaginary, but at the present time evidence is being brought forward, and from sev- eral sources, which would seem to prove the actual existence of this imaginary medium, the ether. Light is propagated and is the effect of very rapid oscillations or electric polarization of the ether. These oscillations are periodical and transverse to the ray of light or direction of transmission. They are exceedingly rapid altera- tions of the electromagnetic conditions of the ether, which vibrate back and forth, or rotate in a plane at right angles to the direction of propagation. That light is an electric effect is substantially proven from its analogy to the electric waves used in the transmission of wireless 160 OPTICAL PROPERTIES OF CRYSTALS 161 telegraphy. They both travel with the same velocity of 185,400 miles per second, and may be polarized, reflected, defracted, and refracted. The wireless waves are very large, while those waves which our eyes are able to detect as light are very small. The range of our eye as a detector is limited to those waves which fall within the colored spectrum ; but that waves, both smaller, as the ultra-violet waves, and larger, as the infra-red waves, do exist we know from other detectors, and our eyes are not able to recognize these waves as light. The ether pervades all space, both the interstellar and the intermolecular, and penetrates even within the atom itself, filling the space between the electrons which com- pose it. The interatomic space is probably as accessible to the ether as the space within a stack of bird cages is to the air, and yet light travels faster in a vacuum than in space filled by a gas or a transparent solid. It is through this modification of the velocity and vibration of the light wave, as it passes through a substance, that the optical properties of any particular crystal become appar- ent. Light may be considered as transmitted through a given medium by means of waves set up in the ether. The periodic changes which constitute these waves take place at right angles to the line of propagation, and in this respect they are known as transverse waves vibrating back and forth in all planes across the line of direction of transmission. The ray is a term conveniently used to denote the direction along which the wave advances. As an illustration of the terminology of wave and wave motion it is best to select, as an example, one in which there is possibly no imagination required, as is the case of the wave motion on the sur- face of water. FIG. 309. In Fig. 309, the position of any particle of water, as a, on the sur- face will determine the wave surface at that point ; as a falls to- ward the arrow, the water surface falls and the wave passes on until a reaches a maximum depression x, when the valley of the wave x is formed ; then a rises until it reaches a maximum position above 162 MINERALOGY the arrow at y, when at that instant another crest is passing the point y. Each time that a completes the path between x and y, and returns to its original position, moving in the same direction, an entire crest and trough have passed, or one wave length, denoted by A. The wave length is the distance oo, measured between the paths of the two particles aa, occupying the same position in regard to the arrow and traveling in the same direction; such particles are said to be of like phase. The period is the time that is taken by any particle to complete the swing back and forth and to return to its original position and condition. The amplitude is the distance oy = ox from the median line to the highest point in its path. It is also to be noted that the particle a moving back and forth along the path yx is not carried forward along the arrow, but like a block of wood rises and falls on the waves. A wave front is formed by the particles or points which are in- fluenced simultaneously as each wave passes them; they are all in the same phase and form a surface or line at right angles to the direction of transmission, at any particular point. When the wave surface is a curved surface, the plane tangent at any particular point will be at right angles to the ray or the direction of transmis- sion at that point. All the above terms are equally applicable to the light wave, but just what the change of conditions along the path xy is, is still somewhat in doubt. The intensity of light is proportional to the square of the amplitude, and the color will depend upon the length or more correctly on the periodicity or number of vibrations per second of the wave. Deep violet light at one end of the spectrum has a wave length of .000396 mm., while dark red at the other end has a wave length of .000795 mm., or about double that of violet, and the yellow sodium light is about halfway between these two, or .00059 mm. Vibrations larger or smaller than these the eye is unable to detect as light ; but that heat rays do exist above and actinic rays below is easily demonstrated by detectors other than the eye. Light waves of all lengths travel in a vacuum with the same velocity, but they differ in their period, since short waves, as violet, must vibrate twice as quickly as the red waves, which are double their length. Upon entering a transparent medium the velocity of light of all wave lengths is modified ; the extent of this modifica- tion will depend both upon the medium through which the light is traveling and upon the wave length or color of the light ; so that lights of different wave lengths will vary in their velocities upon OPTICAL PROPERTIES OF CRYSTALS 163 passing from one medium into another. Substances differ greatly in the way they transmit light ; one class, known as isotropic sub- stances, transmits light equally or with the same velocity in all direc- tions. If a point within such a transparent isotropic substance be imagined as the source of light, the light waves will travel in all directions from this point with the same velocity, and if it were possible to stop the wave at any instant, say after an inch had been traversed from the point of emission, the extreme wave front would be a sphere of an inch radius. Every point of the surface would be one inch from the source of light, and each ray would have traveled exactly the same distance, whatever the direction. Isotropic substances include all gases, most liquids, amorphous solids, as glass, and crystals of the isometric system. Solids, how- ever, when under stress or strain and which under normal conditions are isotropic may show anomalies and apparently belong to the second class, or anisotropic substances ; in which the wave front is not a sphere and the velocity of the transmitted ray will vary with the direction. In anisotropic substances the velocity of light will vary with the direction in which the light is traveling, but in parallel directions within the same medium the velocity will be the same. The anisotropic class includes 'the tetragonal, hexagonal, ortho- rhombic, monoclinic, and triclinic systems of crystals, and also iso- metric and isomorphous solids when under stress or strain, as well as those liquid crystals which show double refraction. The wave front in anisotropic substances is not a sphere, but its form will depend upon the substance. When light strikes the surface of a transparent substance, as glass, it is modified in several ways: (1) some is reflected; (2) some is refracted ; (3) some is polarized ; (4) some is absorbed or lost as light, as it is transformed to energy of another kind. All four effects will depend upon and will vary with the nature of the surface, the angle of inclination of the ray, and the substance. Reflection. If from the head of the arrow, in Fig. 310, a ray of light is traveling in the direction ao, it will strike the surface ss' at the point o. The reflected portion will travel along and in the direction of oa' with a velocity which is unchanged. The two direc- tions ao and a'o will be symmetrical in regard to the normal no at the point of incidence, and will lie in the same plane. The angle aon is the angle of incidence = i = the angle noa', the angle of re- flection. The ray from b, traveling in the same direction and with 164 MINERALOGY an equal velocity as that from a, will strike the surface at point P and will be reflected in the direction of Pb'. When the wave front is at a', the ray from b will be at the point V, for the path apa' is equal to the path bPb', and the arrow will appear as at a'b', but in a reverse position. If the surface of reflection is a truly plane sur- face, the image of the arrow at a'b' will be of the same size and shape. Refraction. When the ray, in Fig. 310, from a strikes the surface at o, a portion of the light, depending upon the angle of incidence and the character of the substance, is transmitted or pene- trates the second transparent medium. The velocity and direction of the entering light will be changed ; the amount of charge will depend upon the medium and the wave length of the entering light. Suppose the upper medium to be air and the lower medium to be water ; light travels approximately three quarters as fast in water as in air. The ray ao, in Fig. 310, will be on the point of entering the water at o, when the ray from b is at the point b" ; while b is traveling the distance b"P, a will have traveled three quarters of this distance in water. To find the wave front of the refracted rays : draw bP parallel to ao, then with o as a center and radius equal to three quarters of b"P draw the circle dT ; from P draw PT tangent to the circle dT and PT will be the direction of the wave front of the refracted rays. When b reaches bi, a will be at ai and the arrow will appear at aibi, enlarged, but not reversed, as is the case with the reflected rays. A man standing on the bank will OPTICAL PROPERTIES OF CRYSTALS 165 appear, to a fish in the water, one and one half times taller than he really is ; while the fish will appear smaller, as the rays follow the same paths in the reverse direction. The angle aio^ = r = the angle of refraction. In passing from a rare medium to one which is more dense, the ray is bent toward a perpendicul ar ; and in passing from a dense medium to one which is less dense, the ray is bent from the perpen- dicular. The angle of refraction will vary with the angle of incidence, but there is always a relation, as the value of is a constant. smr In Fig. 310, in the two right-angled triangles ob"P and oTP, the side oP, or hypothenuse, is common to both triangles. The angle b"oP = noa = i, the angle of incidence, and TPo = Toni = r = the angle of refraction. Pb" = v, the velocity in air, and, oT = vi, the velocity in water ; then Pb" = oP X sin b"oP = oP X sin noa = oP sin i, or v = oP X sin i ; .oT = oP X sin TPo = oP X sin Ton = oP X sin r, or v' = oP X sin r, v sin i or As the velocity of light of the same wave length is, in water, always y the same, no matter what the direction, and likewise for air, , , sin i and - - are constants, sin r The ratio - = n, the index of refraction of the water. When sinr air is taken as the unit of comparison, and the velocity of light in air is one, n, the index of refraction of water, is 1.333. An isotropic substance has a constant index of refraction, what- ever the direction of the path of the transmitted ray may be, and for water the index is 1.333. The indices of refraction of a few other liquids and solids at room temperature and for yellow light are as follows : Ether 1.356 Turpentine 1.472 Benzene . . 1-502 166 MINERALOGY Oil of cedar . . . . . ........... 1-520 Oil of cloves ..... ....... V L54 Canada balsam ...... . ; > . . . . * . 1.548 Carbon bisulphide . . . ... . ..-..".*. -.-. 1.627 Methylene iodide . . . '-..'.:.. . . : . . . . , . 1.742 Fluorite .... . . . V v . ^ I . : . . ..;..;> 1.423 Potash alum ..... ; V;:. :. .> . . .. . . -. 1-456 Crown glass . . . . . .. > -. * 1-515 Rocksalt . .... ..''. ..''.'.*.- . - 1-544 Garnet. . ... . . . . .. ^ - . ... .'. . 1.807 Spinel (Chrome) . . . . . - . ...... . 2.096 Diamond . . . . . . . . . . . ..';>"" . . 2.467 Proustite .... ...... ....'.,... . 3.08 The index of refraction of any substance is different for light of different wave lengths, and also varies slightly with the temperature; as regards the wave length, it is inversely as X 2 , or for light of long wave length, as red, n the index of refraction is less and the angle of refraction r would be greater than in case of violet light, with a short wave, or the violet ray would be bent more in entering the water, Fig. 310, than the red ray. The violet ray would lie nearer the normal than the red ray. This division of white light into colors is known as dispersion. The relative velocity of light in any substance is the reciprocal of the index of refraction, as, n = ^7 ; where v, the velocity in air, is Violet light, which has a larger index of refraction in water, will travel more slowly than red. Critical angle. In the equation, n = -. , sin i may have any value between zero and one ; when all values are considered, there are two special cases, those of the limiting values, o and i. In the case when sin i = i, or the angle of incidence is 90, Fig. 311, then sin i i n = sin~r r S * n r = ~ ' su bstituting the value of n in case of water; sin r = - , or the angle r is 48 36'. ^ -333 When light is traveling along the surface SS', that entering at any point o will take the direction oai, in which the angle aioni is 48 36'. OPTICAL PROPERTIES OF CRYSTALS 167 This is the maximum value of r for water and air, and is termed the critical angle, or the angle of total reflection; for if a ray, as oan, should reach the surface at the point o, in which the angle ano^ is greater than 48 36', the critical angle for water and air, there is no possible value for sin i and no light could pass out of the water into the air, but all is reflected back along the direction of oa m . Viewed from a m un- der suitable condi- tions, there will be a light field outside of the critical angle of 48 36 ' and a dark field in- side of this angle, as in- dicated by the circle in the figure. The di- viding line between these two fields will measure the critical angle. By measuring the critical angle of any substance, its index of refraction is easily determined, as n = j-^ ; - . sin (of the critical angle) The determination of the index of refraction by the total refrac- tometer is based upon this principle. The minimum value of sin i is o ; then n = - becomes zero, smr or there is no refraction, and light passing in the direction of the normal to the surface is not refracted. All isotropic substances have but one index of refraction, for the reason that light is transmitted with the same velocity in all direc- tions ; the wave front is a sphere. In anisotropic substances there are two and even three indices of refraction, and the velocity of light varies with the path followed through the crystal. The wave front is no longer a sphere, as in isotropic substances, but its shape and curvature will depend upon the substance. The wave fronts in anisotropic substances are surfaces all of which agree in being symmetrical to three planes of symmetry at right angles, as the axial planes of the orthorhombic system. These three planes intersect each other in three straight lines at right 168 MINERALOGY angles to each other. Each of these lines represents a direction parallel to which there is a maximum or minimum index of refrac- tion or velocity, for transmitted light. The relative length of these axes will also represent the relative speed of transmission, remem- bering that the velocity is the reciprocal of the index of refraction. Double refraction. A ray of light upon entering an anisotropic crystal or substance, in general, travels with two different velocities within the crystal, or it is broken into two rays, each of which pos- sesses its own index of refraction. In other words, one is a slow ray, the other is a fast ray. The difference between the. values of the indices of refraction of the two rays is a measure of the birefrin- gency of the substance in that direction, for the birefringency or double refraction varies with the direction of transmission. For calcite one index of refraction = 1.658 and the other = 1.486 ; as these are the maximum values, or represent the maximum difference between the two indices, their difference, or .172, would be the double refraction of calcite ; which is very high or strong. In most minerals it is represented by a small figure in the second decimal place, or even in the third, as that for quartz is .009 and that for Orthoclase is .007. Since calcite is an example of birefrin- gency in an exaggerated degree, and it is transparent and easily obtained, it is an extremely good mineral with which to demon- strate this peculiar property of crystalline substances. The usual cleavage piece of calcite is a rhomb in shape. If such a cleavage piece of calcite be placed over a pinhole in an opaque paper and then held up to the light, two pinholes will appear, Fig. 312 ; one will be seen above and nearer than the other; this is due to the difference of the velocities of the two rays. The dis- tance between the two images will depend upon the thickness of the cal- cite. When the rhomb is revolved, one image e, Fig. 313, will appear to revolve around the other, or that ray is refracted to a greater extent than is the other ray. In fact, when the ray of light enters the calcite at right angles to the surface and the eye is in the direc- tion of this ray, when the rhomb is revolved one image is sta- FIG. 312. OPTICAL PROPERTIES OF CRYSTALS 169 tionary, and this is what would be expected if the crystal were an isotropic substance, as there is no refraction when the ray falls normal to the surfaces. This ray follows the ordinary law and is therefore termed the ordinary ray. Its index of refraction is written co. In the case of calcite the index measured with monochro- matic sodium light (yellow) is written, co y = 1.658. The second ray follows another law which is entirely different from that of the ordinary ray, and its velocity and therefore its index of refraction (written c) will vary with the di- rection ; this ray is known as the extraordinary ray. The index of refraction taken at its maximum difference from that of the or- dinary ray and for sodium light is written y = 1.486. When the index of refraction of the extraordinary ray is smaller than that of the ordinary ray, or the extraordinary ray is the fast ray, o, the crystal is said to be optically negative, written ( ) as in calcite. In quartz, where co, it is optically (-f), and the extraordinary ray is the slow ray. All crystals of the tetragonal and hexagonal systems have two indices of refraction ; one, that for the ordinary ray, is constant for all directions in the crystal, as in isotropic substances ; the other, that for the extraordinary ray, varies with the direction in the crys- tal, from the value of the index of refraction for the ordinary ray as one limiting value, to a maximum or minimum as the other limit, according to the ( ) or (+) character of the crystal. Wave surfaces in hexagonal and tetragonal crystals. In Fig. 314, if any point within a hexagonal or tetragonal crystal, as o, be illuminated, and act as the source of light for the smallest frac- tion of a second, that portion illuminated will be bounded by the wave front. Its distance from the source of light o, in any direc- tion, will depend upon the velocity with which the ray travels through the crystal in any given direction. At the end of any short period of illumination the ordinary ray to has traveled the distance ox; as the ray travels with the same velocity in any and all direc- tions, the circle with o as a center and a radius ox will represent the 170 MINERALOGY -c FIG. 314. Negative. section of the spherical wave front, in the plane of the paper. The extraordinary ray travels with a velocity which varies with the direction; the minimum value of which, let it be supposed, is in the direction of the c axis and is equal in this direction to that of the ordinary ray ox. They will both arrive at p and p' on the c axis simultaneously, or for light traveling through the crystal parallel to the c axis there is only one in- dex of refraction. Crystals of the tetrag- onal and hexagonal systems are isotropic in the direction of their c axis only; such crystals are optically uniaxial. The direction in which the extraordinary ray travels with a maximum velocity is at right angles to the c axis or paral- lel to it according to the optical sign. Let it be sup- posed the crystal is calcite (-), the maximum velocity will therefore be in the plane of the lateral axes, or the basal plane; this is true for any direction in this plane from o. Let this maximum value be represented by oa. The cross section of the wave front parallel to the basal plane is a circle. Inter- mediate values between po and oa, as in the direction of od or of, when plotted on the plane of the paper, form an ellipse, which is similar for all plane sections containing the c axis Ihe whQle wave front of the extraordinary ray is an ellipsoid of revolution, the axis of which is the c axis, or is parallel to the optic Ihpsoid of revolution or wave front of the extraordinary FIG. 315. Positive. OPTICAL PROPERTIES OF CRYSTALS 171 ray is tangent to the sphere or wave front of the ordinary ray at two points p and p', where the crystallographical axis c cuts them. The sphere in this case, that of calcite, an optically negative crystal, is entirely inclosed by the oblate ellipsoid. In the case of quartz, an optically positive crystal, the wave front of the extraordinary ray is represented by a prolate ellipsoid of revolution, which is in- closed within the circle or sphere, as represented in Fig. 315. Optically biaxial crystals. The wave front in crystals of the orthorhombic, monoclinic, and triclinic systems is not an ellipsoid of revolution, but a combination of two wave surfaces, one within the other, continuous at four depressions, Fig. 316, or symmetrical points-, the position of which depends upon the relative values of the three indices of refraction. This fourth dimensional surface is, however, symmetrical to three planes of symmetry intersecting each other at right angles, in three straight lines, analogous to the axes and planes of the orthorhombic system. The three lines of intersection always represent directions within the crystal parallel to which there is a maximum or minimum velocity of light, as all such crystals have three indices of refraction. They are repre- sented by a, p, and 7. The mean index of refraction is and y a will always represent the greatest double refraction, as "Y is the greatest and a the smallest index. FIG. 316. FIG. 317. Sections of the wave front in the three planes of symmetry are represented in Figs. 316, 317, 318. It will be noted that in each case there is a circle and an ellipse, or for each of these sections there are 172 MINERALOGY two rays, one of which has a constant index of refraction within the plane and is therefore an ordinary ray ; the other, represented by the ellipse, is variable in its velocity, and is an extraordinary ray. In Fig. 317 the radius of the large circle represents the maximum velocity I, and the inner ellipse the variable ray with its two limiting values a ^ and - ; Fig. 318 represents the conditions in the plane of sym- metry at right angles to a, in which the inner circle is the minimum velocity - and in which *y is the ordinary ray. The diameters of the ellipse represent the variable rays ; in Fig. 316 the conditions -in the third plane of symmetry are represented, or the intermediate value I represents the velocity of the constant ray, the circle, and the el- lipse represents the maximum velocity - and the minimum velocity - in the crystal ; in this section the ellipse and circle cut each other at the four points P, P', P", P'", which represent the de- pressions in the wave surface and are the four points at which the inner and outer surfaces are con- tinuous. The two directions PP" and p'p'" are the two optic axes. Parallel to these two directions there is no double refraction, and the section of the wave surface perpendicular to the optic axis in each case is a circle, as was also the condition in uniaxial crystals. Strictly these two lines PP" and P'P'" are the secondary optic axes, but the true optic axes are so near them as not to be separable from them in practice. In any section of the wave front other than in the three planes of symmetry and to an optic axis, light will travel with two velocities or rays, neither of which will be an ordinary ray, but both will vary m speed with the direction of transmission. t is seen, Fig. 316, that the two optic axes, the direction of the greatest velocity, and at right angles to it the direction of the least FIG. 318. OPTICAL PROPERTIES OF CRYSTALS 173 velocity, are all contained in the one plane ; this plane is known as the plane of the optic axes or axial plane, abbreviated to (Ax. PL). The line bisecting the smaller angle between the optic axes is the acute bisectrix (BxJ or first median line ; the line bisecting the larger angle is the obtuse bisectrix (Bx ). The internal angle between the acute bisectrix and the optic axis is represented by V and 2 V = pop ; , the angle between the optic axes, always less than 90 and measured within the crystal. When measured in air the angle is designated 2 E ; 2 E owing to refraction is always larger than 2 V and is often 180 from total reflection. The value of 2 V varies with different substances and will depend upon the indices of refraction. When the three indices are known, the angle 2 V may be calculated from the formula: tan V = As the indices of refraction vary with the wave length or color of light, it will be seen that 2 V for violet light will differ from the value of 2 V for red light ; their dif- ference will measure the disper- sion of the optic axes. The optical sign of biaxial crystals. The intermediate in- dex of refraction p in different biaxial crystals may vary from a as a minimum to -y as a maxi- mum limit. In Fig. 319, as the value of p decreases the circle ycy' will ap- proach the circle yxy' and the radius oc will approach ox. (The figure is drawn with the three axes ox, oy, and oz proportional to the indices of refraction.) The four points marked c will draw nearer to x, while the optic axes, perpendicular to these circular cross sections, will also draw nearer each other, constantly decreasing the angle 2 V, pop' ; when p = a, c reaches x and p, p' reaches Z ; in which case the angle between the optic axes is zero and the cross FIG. 319. 174 MINERALOGY section is a circle, and the ellipsoid is one of revolution, with z as the axis of revolution, analogous to the prolate ellipsoid in quartz. In such cases, where z is the acute bisectrix and the value of p is nearer to a than to y, the crystal is said to be optically (+). On the other hand, when the point c moves up toward z, the value of p will increase, and the angle 2 V will increase constantly until it is greater than 90, when the line oz will be the obtuse bisectrix and ox the acute bisectrix ; when c reaches z, the ellipse will be an oblate ellip- soid of revolution, analogous to that of calcite, and the crystal is said to be optically negative ( ). The three axes of the ellipsoid are usually written X = a = a, Y = b = P, Z = c = "Y. The relations of the axes of the ellipsoid to the crystallographical axes in the orthorhombic, monoclinic, and triclinic systems vary with the possible conditions, de- pending upon the symmetry of the system and the relation of the axial plane of the ellipsoid to the planes of symmetry in the system. In the orthorhombic system, where the three crystallographical axes are at right angles to each other, these correspond in direc- tion to the axes of the ellipsoid, and the position of the planes of symmetry of the ellipsoid is fixed parallel to the planes of symmetry of the system. The axes X, Y, or Z may correspond with any one of the crystallographic axes, but for any one species this relation is definite, as is shown in Fig. 320, a diagram of the optical conditions in the mineral aragonite, where the plane of the optic axis is parallel to the macropinacoid (Ax. PI. = 100). The acute bisectrix is X = 6, the crystal is therefore ( ) ; b = Z, a = Y; 2V= 18 11'. In the monoclinic system, the plane of symmetry of the system is parallel with one of the planes of the ellipsoid and the orthoaxis b is parallel to one of the axes of the ellipsoid, this axis is therefore fixed ; the other two must lie in the plane of symmetry of the sys- tem; but their relation to the a or c crystallographical axes will vary with the mineral species, and their relation is characteristic FIG. 320. Diagram of the Optical Properties of Aragonite. OPTICAL PROPERTIES OF CRYSTALS 175 of the species. The axial plane may hold one of two positions : (1) parallel to the plane of symmetry of the system ; and (2) at right angles to it. Figure 321 represents the optical conditions in the mineral wollastonite. The axial plane is parallel to 010 (Ax. PI. = 010), with X as the acute bisectrix (Bx a = X) , optically ( ). The angle between the acute bisectrix and the axis c is 32 12' in the acute angle p, or ex- pressed (Bx aA c = 32 12' be- hind) ; 2 V = 40. In the triclinic system, where, at most, there is only a center of symmetry, there is no relation between the optical ellipsoid and the crystallographical axes, but usually the plane of the optic axes is fixed in any given mineral species. In the description of the optical properties of the triclinic minerals the plane of the optic axes is located by measuring the angle between its trace and some convenient edge, or by any convenient method. In the case of axinite, the acute bisectrix is normal to 111. The trace of the plane of the optic axes on 111 makes an angle of 40 with the edge 111/110, and 24 40' with the edge Ill/Ill. POLARIZED LIGHT In ordinary light the vibrations are not restricted to any one plane, as the plane of the paper, in Fig. 322, but take place in all possible planes inter- secting in the ray as an axis, thus the vibra- tions of the ordinary beam of light are very complex. When such a complex ray strikes the polished surface of a transparent sub- stance, a .portion of both the reflected and 176 MINERALOGY the refracted ray is modified and the vibrations of the modified portion are restricted to one plane. The amount of this modified light will depend upon the angle of incidence, the character of the surface, and the substance. Light in which the vibrations take place in one plane only is termed polarized light, or plane polarized light ; when the vibrations are in circular orbits, circular polarized ; and when they are in elliptical orbits, elliptically polarized. Both the reflected and refracted ray are completely polarized when the angle between them is 90, or, as Brewster's law expresses it, tan (angle of polarization) = n (the index of refraction). In case of rock salt, n = 1.544, or the angle of polarization would be 57 5' ; when, in Fig. 322, the angle noR = 57 5' in case of rock salt and air, the angle R'oRi would be 90 and both the reflected ray oR' and the reflected ray oRi are completely plane polarized. The vibrations in the reflected ray take place at right angles to the plane of the paper and the ray is said to be polarized in the plane of the paper, parallel to the plane of incidence RoR'. In the re- fracted ray, the vibrations take place parallel to the plane of the paper, and it is said to be polarized in the plane perpendicular to the plane of the paper, and at right angles to the plane of incidence. The two rays after polarization are vibrating in planes at right angles. This is the condition in all isotropic substances. In anisotropic substances, in case of refracted light, both the ordinary and extraordinary rays are completely polarized, their vi- bration planes are at right angles and rigidly fixed by the molecular arrangement of the crystal. In a cleavage piece of calcite, Fig. 323, when the four sides of the rhombic faces are approximately equal, the ordinary ray o upon emerging will be vibrating parallel to aa', its plane of polarization will be parallel to the short diagonal cc' ; the extraordinary ray e will vibrate parallel to cc', its plane of polarization will be parallel to the long diagonal aa', and, furthermore, it is impossible for light to emerge from the calcite^the vibrations of which do not conform to either of these two directions. The two vibration planes and planes of polarization are rigidly fixed by the crystalline structure of the calcite. OPTICAL PROPERTIES OF CRYSTALS 177 If in any case, or by any means, one ray, either the o or e ray, could be absorbed, light passing out of the fragment would be vi- brating in one plane only. It is a property of plates of tourma- line that when cut parallel to the c axis they absorb one ray, the ordinary, and the extraordinary ray, vibrating parallel to the c axis only, is transmitted. All light transmitted by such a section of tourmaline is vibrating in one plane, that parallel to the c axis. Such a section of tourmaline, or any other device, used to produce polarized light is termed a polarizer, Fig. 324. When the light, as transmitted by the polarizer, is viewed through another similar section of tourmaline, it will be observed -a - c FIG. 324. Tourmaline Polarizer. at once that the intensity depends upon the relation of the two sections of tourmaline. When the two c axes of the sections are at 90, as in Fig. 325, no light will be transmitted by the second section in this crossed position, and the condition will be that of darkness ; the amount of light transmitted by the second section, termed the analyzer, constantly increases from zero in the crossed position to a maximum, when the 6 axes of the polarizer and analyzer are parallel. The analyzer allows no light to pass, the vibrations of which are at right angles to its vibration plane ; as all light pass- ing the polarizer is thus vibrating, no light can pass, when the two sections are in the crossed position, and darkness is the result. As the analyzer is rotated, the amount of light passing increases to 178 MINERALOGY a maximum when the two vibration planes are parallel, and all the light passing the polarizer is transmitted by the analyzer. Intermediate positions are explained as in Fig. 326 ; let PP' be the vibration plane of the polarizer and AA' that of the analyzer, be the amplitude and plane of vibration of light passing the polarizer; according to the parallelogram law in mechanics this wave may be divided into two waves vibrating at right angles to each other, one ce parallel to the vibration plane of the analyzer and with an amplitude ce, the other vi- brating at right angles to it and with an amplitude eb ; the ray represented by ce passes the analyzer, while eb at right angles to it is extinguished. The amplitude of the transmitted ray bf, which illuminates the field of the analyzer, increases from zero in the crossed position, where all light is absorbed, to cb in the parallel position when all the light is transmitted. Two such sections when mounted in a holder are known as the tourmaline tongs, and may be used to test the double refraction and the vibration planes of light in any mineral section placed between them. If the light reflected from a polished table top is viewed through one of the. tourmaline sections, as an analyzer, on revolving the section the intensity of the transmitted light will be greatest when the long or c axis of the tourmaline section is parallel to the table OPTICAL PROPERTIES OF CRYSTALS 179 fre FIG. 329. Tourmaline Analyzer. top, and least when at right angles to it, showing that some of the reflected light is polarized and that the plane of vibration of the polarized reflected light is parallel to the table top and at right angles to the plane of incidence. In testing the vibration planes of the two rays transmitted by the calcite rhomb, when the vibration plane of the tourmaline section is parallel to the long diagonal of the rhombic face of the calcite, Fig. 327, only the ordinary ray will appear ; its vibration plane must therefore be parallel to this diameter. Upon revolving the tourmaline section, both rays ap- pear and are equal in intensity after a revolution of 45, Fig. 328. Upon revolving the tourmaline 90 only one ray will appear, the extraordinary ray, the vibration plane of which must therefore be parallel to the short diagonal of the calcite face, Fig. 329. The two rays are polarized and their vibration planes are at right angles ; this is true of all anisotropic substances. The nicol prism. As polarized light is necessary in the study of the optical properties of minerals, and to avoid the natural color of tourmaline sections, Nicol in 1828 devised the instrument now used as the source of polarized light in most optical instru- ments and known as the nicol prism or nicol. It is constructed of clear colorless calcite in such a manner that one ray, the ordinary ray, is internally totally reflected and ab- sorbed, while only the extraordinary ray emerges, thus yielding plane polarized light, all of which is vibrating in one known plane. Fig. 330 is a section through the short diagonal of a nicol prism, illustrating its construction. A clear, colorless cleavage piece of calcite, three times as long as broad, is cut along the plane PP' per- pendicular to the plane of the diagram, from the obtuse angle at P to that of P'. The angle PP'e should be 22 ; the end surfaces are then cut down until the angles dP'P and ePP' are right angles. The two polished halves are cemented together in their original -posi- tion with Canada balsam, a film of which will separate the two halves and lie along the plane PP'. The cemented calcite is then set in cork, the walls of which next the calcite have been blackened 180 MINERALOGY to absorb any light that may fall on them after being reflected to the sides of the calcite. A ray of ordinary white light on entering a nicol, as at R, is divided into an ordinary and an extraordinary ray, having different indices of refraction and traveling different paths through the calcite; their angle of total reflection will therefore differ. The ordinary ray, with an index of refraction of 1.658 between air and calcite, is refracted more than the extraordinary ray with an index of refraction of 1.486 ; this ray will meet the film of Canada balsam at an angle greater than 69, which is the ap- proximate critical angle of the ordinary ray between Canada balsam and calcite, since o> = 1.6583 divided by the index of refraction of Canada balsam, 1.548, = 1.0712, the index of refraction of the ordi- nary ray as between Canada balsam and calcite. As between these two media, sin s (of the critical angle) = critical an- 1.0712' gle = 68 59'. All ordinary rays meet- ing the film of balsam at an angle greater than 68 59' will be totally reflected in the direction as indicated, and absorbed by the blackened walls of the cork mounting. The index of refraction of the extraordi- nary ray varies with the direction through the crystal, but in this particular direction it is but little different from that of the balsam, 1.548; its path on entering the calcite is deviated much less than the ordinary ray, and on meeting the film of balsam is but little effected, passing through with little or no refraction, and emerging at the opposite end of the nicol as plane polarized light, with a vibration plane parallel to the short di- agonal cc of the rhombic section, and polarized in the plane parallel to the long diagonal PP'. Other styles of polarizing prisms have been devised, either to economize space or calcite, as suitable calcite is very scarce and expensive, since the Iceland OPTICAL PROPERTIES OF CRYSTALS 181 supply has been exhausted. They all agree, however, in the principle of totally reflecting either the ordinary or extraordinary ray out of the field. As usually mounted, the polarizing nicol is under the microscope stage, with its plane of polarization crossing the field, from to 180 on the scale, while the analyzer is mounted in the tube of the microscope in such a way that it may be pushed in or out of the line of vision as required ; its plane of polarization is at right angles to that of the polarizer, or in the crossed position. Interference of polarized light. Whether we speak of light as due to waves, or to the periodic vibrations or change in con- ditions, or whether light is due to an electromagnetic disturbance of the ether, it remains nevertheless true, that one disturbance is FIG. 331. influenced by another and may be added to or subtracted from the other, according to the phase of each. If two waves of the same length are vibrating in the same plane and phase, as the two waves a and b in Fig. 331, but of different amplitudes or intensities, the result is an entirely new wave c with an amplitude oc, or a wave with an amplitude of a + b, and the illumination of the new wave is equal to that of the other two combined. When the waves are in opposite phases, the result is the difference of the two amplitudes, the new wave a, with an amplitude oa equal to oc oa, and the illumination is decreased. Should the amplitudes be equal and the one wave be a half phase or wave length behind the other, their difference would be zero and darkness would result, or one wave is said to interfere with the other. When light of the same wave length or color and of the same intensity, i.e., derived from the same source, is polarized in two rays, these two rays will interfere if brought to vibrate in the same plane. The result of this interference will depend upon the con- ditions of vibration, or how much one wave has been retarded, or is vibrating behind the other. 182 MINERALOGY In Fig. 332, the ray R is reflected at the point o in the direction of oR'. Some of the light, however, enters the medium P and is refracted in the direction oRi and at R] is reflected in the direction of RIO' and passes out of the medium P, in the direction o'R". The light, or ray, o'R" is made up of two rays, one which has trav- eled the path oRio' in the medium P, and a reflected ray iV, which will prob- ably be vibrating in a different phase and there- fore in position to inter- fere. If the refracted ray is retarded 1/2 X due to the differences of paths and velocities of the two rays while the refracted ray is passing through the medium P, then the eye at R" will perceive no light, or darkness will result when monochromatic light is being used. When the medium P is of uniform thickness the relative paths for each ray will be the same for all points and the surface will be uniformly lighted. If the paths within the medium P can be pro- gressively varied, or if the section P is wedge-shaped, as indicated by the dotted line, then the difference of phase between the reflected and refracted rays will increase with the thickness of the wedge, as the in- ternal path fol- lowed by the ray FIG. 333. Diagram of the Quartz Wedge. R will be much shorter than the path followed by the ray 2, and on emerging at o' and o'" will be retarded proportionally. At the edge of the wedge, Fig. 333, where the thickness is zero, there will be no interference or diminution in the illumination. As the wedge thickens, the refracted ray will be retarded more and more behind the reflected ray, with a decrease of the illumi- OPTICAL PROPERTIES OF CRYSTALS 183 nation until the refracted ray is exactly 1/2 a wave length behind the reflected ray, or 1/2 X, then darkness will be the result. From this point the illumination will increase until the refracted ray is a whole wave length behind the reflected ray, when there will be a maximum illumination. There will therefore be bands of light, representing a maximum light at each whole wave length that one ray is retarded behind the other, as at 1, 2, 3, and there will be a band of minimum illumination at points, as 1/2, 3/2, 5/2, at which one ray is retarded an odd number of 1/2 wave lengths behind the other. This condition of alternate bands of light and darkness obtains only when monochromatic light is used ; when white light is used, which is composed of waves of all lengths or colors, and which differ in their velocities in passing through the wedge, their dark and light areas on the wedge will not corre- spond, and the area which will be dark for yellow will be light for red, with a result that the surface viewed with reflected light will show color bands (these bands may be seen on the quartz wedge when held at the proper angle). Beginning at the thin edge of the wedge, all the interference colors will have appeared in order, when the retardation has reached one wave length, or X ; then they are repeated in the same order, when 2 X is reached, and again to 3 X. These color effects due to the interference of light are well illus- trated by the play of colors on soap bubbles ; in the iridescent films of carbonates, oxides, or oil on the surface of water ; in the cleav- age fractures of such a clear mineral as calcite, and in the small internal and irregular fractures of the opal. Order of colors. In the series of colors caused by the inter- ference of light, those which appear first, on the thin end of the wedge, or are caused by a retardation of one wave length or less, are termed the colors of the first order; those from X to 2X, the second order ; and those from 2 X to 3 X, the third order ; etc. Above the fourth and fifth orders the individual colors are not well defined and return to the high order gray. The lower orders of colors are each characteristic in intensity and tone, and with experience may easily be distinguished ; as, for instance, red of the first order, from red of the second or third orders ; since the order of color yielded by sections of approximately the same thickness of the various double refracting minerals is a measure of their double refraction, it is most important that one should be able to recognize the colors of various orders. The most important of these are : 184 MIXER ALO( IV FIRST ORDKK SECOND ORDKK THIRD ORDER Grays Purple Light blue Straw yellow Deep blue Bright green Deep red Light green Yellowish green Light yellow Faint red Bright red FOURTH ORDER Indistinct These colors may be compared by use of the quartz wedge, Fig. 334; those produced by the thin edge are of the first order, starting with gray of the first order. Uniaxial crystals. It has been pointed out that light in pass- ing through a uniaxial crystal is divided into two rays polarized and vibrating at right angles to each other, one of which travels with the same velocity whatever the direction, while the velocity of the other varies with the direc- tion. If an ellipsoid, Fig. 335, be con- structed in which the three axes are drawn proportional to the three indices of refraction, which are proportional to the reciprocals of the three velocities, this ellip- soid, in the case of uniaxial crystals, will be one of revolution, every plane section of which will be an ellipse; there is, however, one di- rection, that perpendicular to the optic axis, in which the plane sections are circles, and light is transmitted in the direction of the optic axis without double re- fraction. The radii vectores of the elliptical section will be a measure of the indices of refraction of the two possible rays passing through the crystal in a path at right angles to the section, and their directions will also indicate their planes of polarization. Let Fig. 336 be a section containing the axis of rotation of such an ellipsoid, in which ox represents the smaller index of refrac- tion and oz the larger. Light traveling in the direction of zz' is transmitted with a velocity proportional to oz and an index of re- fraction of ox; similarly a ray in the direction of xx' is transmitted \ Gray. Straw yellow. Red of the first order. Purple. Blue. Green. Yellow. Red of the second ord< Blue. Green. Yellow. Red of the third order. 1-1 FIG. 334. Quartz Wedge. OPTICAL PROPERTIES OF CRYSTALS 185 FIG. 335. with a velocity of ox and an index of refraction of oz. Such an ellipsoid is known as the indicatrix of Fletcher. Any ray whatever, as the ray entering the crystal at R, Fig. 335, will in general be transmitted as two rays. The indices of refrac- tion will be represented by the radii vectores of the elliptical section of the indicatrix, passing through the point o and perpen- dicular to the direction of the ray, as the ellipse bab', which in uniaxial crystals contains one diameter aa', representing, the ordinary ray; this diameter is constant in all sections of the indicatrix passing through the point o. The two planes Rcob and Raa', at right angles to the elliptical section aba' and con- taining the two diameters bb' and aa', are the planes of vibration of the two rays ; of these the extraordinary ray vibrates in the plane containing the optic axis cc' and the direction of the ray Ro, and termed the principal optic section. The extraor- dinary ray vibrates in the principal optic section and is polarized in the plane ROaa' at right angles to it. There is one direction in which the two diam- eters of the elliptical section are equal, that at 90 to the optic axis, or the ellipse becomes a circle and the two rays are transmitted with the same velocity and with no fixed plane of vibration; they are not polarized. Angle of extinction. When a section of a uniaxial crystal, or in fact any double refracting substance with plane parallel faces, is 186 MINERALOGY examined between crossed nicols, it will be found, on rotation of the section between the nicols, that the light will be entirely extin- guished, or decrease to a minimum illumination, at every 90, and the section will be dark. From the point of darkness the illumi- nation increases constantly upon further revolving the section, until a maximum is reached at a point 45 from the point of dark- ness, and then decreases to a minimum after a revolution of the section through another angle of 90 ; these conditions are repeated four times in the complete revolution of 360. Interference of polarized light in passing mineral sections. Let Fig. 337 be such a section; then light entering the section will be transmitted as two rays vibrating in planes at right angles to each other. Let ee'oV represent the elliptical sec- tion of the indicatrix; the two rays will leave the section vibrating in the planes ee' and oo' ; also let PP' and AA' be the vibra- tion planes of the polar- izer and analyzer. If RO represents the amplitude and the direction of the vibrations of the plane polarized ray passing the polarizer, then on entering the section this ray will be resolved into two rays, ooi, vibrating parallel to o', and oe, vibrat- ing parallel to ee'. When the ray oo f enters the analyzer one com- ponent oo n vibrating parallel to the vibration plane AA' of the analyzer passes, and passes without diminution, while the other component, vibrating parallel oiOn at right angles to AA' having no component in the plane AA' is extinguished by the analyzer. The two rays oo n and oe n , vibrating parallel to AA' and therefore in position to pass the analyzer, are also in position to interfere, and the resultant light depends upon this interference. When white light is used, the resulting interference color will depend upon the double refraction of the substance; upon the direction of the section in the crystal; and upon the thickness of the section. When monochromatic light is used and one ray is retarded behind FIG. 337. OPTICAL PROPERTIES OF CRYSTALS 187 the other one wave length in passing the section, or any multiple of whole wave lengths, oo n will be opposed to the vibrations of oe n and there will be darkness during a complete revolution of the section. The conditions are the same as in the quartz wedge, but here the half wave of the nicol is added. As the vibration planes of every mineral section are absolutely fixed, they may be determined, and if necessary their traces marked, on the section ; if the section is revolved until there is a minimum amount of light or darkness, as viewed through the analyzer, be- tween crossed nicols, then the traces of the vibration planes of the section will be parallel to the vibration planes of the analyzer and polarizer or to the cross hairs in the eyepiece. One of these planes is the principal optic section or contains the optic axis, which in uni- axial crystals is parallel to the c crystallographical axis. It follows that in all sections through the crystal parallel to the prism zone one of the vibration planes of the section will be parallel to pris- matic or pinacoidal cleavage cracks in the section, or at right angles to them. Darkness will occur on viewing the section in the microscope when one of the cross-hairs is parallel to the cleavage cracks; the section under these conditions is said to possess parallel or straight extinction. The extinction angle of any section is measured by the cross hairs in the eyepiece of the microscope. They are set parallel to the vibration planes of the nicols ; t^hen when extinction occurs on revolving a mineral section on the stage, the vibration planes of the section are parallel to the cross hairs. A reading is taken from the graduated circle on the stage, then the stage is turned until the cleav- age crack is parallel to the hair, when another reading is taken ; the difference between these two readings will be the extinction angle of the section, All sections of uniaxial crystals, not parallel to one axis of the ellipsoid, extinguish at angles other than 90. The extinc- tion angle will vary with the inclination of the section, but extinc- tion is always symmetrical, or divides the angle between cleavage cracks equally. In basal sections of uniaxial crystals there are no definite vibra- tion planes, and the light passed by the polarizer will pass through the section unchanged, to be extinguished by the analyzer, and the field will remain dark during a complete revolution, as if there were no section at all between the nicols. Determination of the slow ray. The slow ray may be deter- mined by means of the quartz wedge. This is cut from a crystal 1SS MINKKALOCJY of quartz in such a manner that one flat side is parallel to a plane containing the optic axis, i.e., the vertical crystallographical axis; the long edge of the wedge, in most cases, is inclined at an angle of 45 to the optic axis. Some wedges are cut with their long edge parallel to the optic axis. In all cases the vibration plane of the slow or extraordinary ray is always indicated on the wedge by an arrow or mark, as in Fig. 334. In the tube of all petrographical microscopes, just above the ob- jective, is a slot, into which the quartz wedge slips back and forth, in such a position that the vibration planes of the wedge are fixed at 45 to the vibration planes of the nicols. A section in which the vibration plane of the slow ray is to be determined is placed on the stage of the microscope and revolved to extinction, then placed at 45 from this position, when the vi- bration planes of the section will lie at 45 to the vibration planes of the nicols and will be parallel to those of the quartz wedge when in position. At this 45 position the section will be evenly colored. Minerals in rock sections are evenly ground to approximately .03 mm. in thickness, and when interference colors of individual species are given they refer to sections of about this thickness. The- color will depend upon the thickness of the section, the direction of the section in the crystal, and the double refraction of the sub- stance. As an illustration let it be supposed that the section yields a red of the first order. First order red may be obtained by using a quartz wedge as a section on the microscopic stage, pushing it under in the 45 position until the first red is obtained. A second wedge is now pushed in the slot of the microscope above the objec- tive ; as the edge of the second wedge enters the field of vision there will be a change of color noted. Whether the change of color goes up the scale, from red of the first order to purple, blue, green, etc., of the second order, or down the scale to yellow and grays of the first order, will depend upon whether the difference between the vibrations of the slow ray and the fast ray is still increased by the second quartz wedge or decreased. If the slow ray of the section or the first quartz wedge used as a section is parallel to the slow ray of the second wedge (the direction of each is marked on the wedge) which is inserted in the tube of the microscope, the color change is up the scale, or the effect is that of thickening the section. When the vibration plane of the slow ray of the section is at right angles to that of the quartz wedge, upon pushing the wedge in slowly the colors will go down the scale, from red to yellow and gray, OPTICAL PROPERTIES OF CRYSTALS 189 and finally a shadow will appear, or darkness, at which point the difference between the slow and fast rays of the section is exactly equal to that of the quartz wedge, and the wedge is said to com- pensate the section ; for this reason the wedge is often termed a compensator. When compensation occurs, if the section is re- moved from the stage, the quartz will show the original color, due to the double refraction of the mineral section before the wedge was inserted. At the point of compensation, if the wedge is pushed farther through, the colors will rise in the scale unin- terruptedly to the end of the wedge. When the direction of the c axis in the section can be determined, either from cleavage cracks or crystalline edges, and the vibration plane of the slow ray is known, then the optical sign of the section is also known ; for when the c axis is parallel to the long edge of the quartz wedge and the slow ray parallel to the slow ray of the quartz wedge, the optical sign is the same as that of quartz (+) ; when the slow ray is at 90 to the slow ray, as marked on the wedge, the sign is opposite to that of quartz ( ) . Pleochroism is the unequal absorption of light waves of different lengths. In the case of tourmaline, when the section was thick enough it absorbed all the light vibrating parallel to the basal section and therefore the section appeared dark for light polarized and vibrating only in this direction. When one color or wave of one length is absorbed more than another, the color of transmitted light will change with the direction, or plane of vibration. Miner- als in which pleochroism is well marked will appear differently col- ored according to the vibration plane of the transmitted light. The absorption reaches a maximum when the vibration planes are parallel to the planes of symmetry of the indicatrix. In uniaxial crystals there can be only two directions, parallel to the c axis, and parallel to the basal section. Crystals of this class can show only two maximum absorption directions and are said to be dichroic. In biaxial crystals there are three maximum directions possible, and these are said to be trichroic or pleochroic. To test a section for absorption or pleochroism, it is placed on the stage and revolved to extinction, then the analyzer is removed and the color of the section noted ; it is then revolved 90 and the color again noted. Any difference in color is due to the unequal absorption of the two rays vibrating in the section, as in the first position one vibration plane of the section is parallel to the vibra- tion plane, of the polarizer and transmits the light, while in the 190 MINERALOGY second position the second vibration plane is parallel to the plane of the polarizer and transmits the light. If it is darker when the extraordinary ray is passing than when the ordinary is passing, and the difference is not marked, then it is noted, absorption or pleochroism is weak, c < o>. If there is a change of color, this is also noted thus : = green, . Circular polarization. When the interference figure of a sec- tion of quartz of 3.5 mm. in thickness cut at 90 to the optic axis is viewed in the microscope, it will be noted that the central por- tion is not dark, as would be expected, but colored, and the dark cross is not continuous through the central portion of the field, Fig. 344. This is caused by a peculiar property of most crystals belonging to the holoaxial types, of rotating the plane of polarized light. When the light from the polarizer enters such a section, cut at 90 to the optic axis, it is broken up into two rays cir- cularly polarized in opposite di- rections and one traveling faster than the other. The two rays on emerging from the section unite to form plane polarized light, but as one ray was faster than the other the plane of the FIG. 344. Interference Figure of Quartz. ,,. , i i resulting plane polarized ray is not the same as that of the ray on entering the section, but it has been rotated through an angle, the size of which will depend upon the thickness of the section, the specific rotating power of the substance and the color of light used. The plane will be rotated to the right (clockwise) in right-handed crystals and to the left (anticlockwise) in left-handed crystals. If monochromatic light is used to make this observation, in the ordinary section the central portion of the field is dark, as the light passing is still vibrating in the plane of ^he polarizer and is ex- tinguished by the analyzer ; but in case of quartz the central por- tion of the field is illuminated, as the plane of polarization has been rotated through an angle and the analyzer no longer extinguishes it. In order to do so, the analyzer must be rotated through the same angle, clockwise in right-handed crystals and anticlockwise in left-handed crystals, through an angle, other things being OPTICAL PROPERTIES OF CRYSTALS 197 FIG. 345. Airy's Spirals. equal, depending upon the wave length or color of the light. In the case of white light a right-handed crystal, when the analyzer is rotated t clockwise, and to the right, the central portion of the field passes from red to orange, yellow, green, blue, violet, or will go down the scale of colors. In a left-handed section, this order of colors is yielded by a rotation of the analyzer to the left or anticlockwise. When a right-handed section is superimposed on a left- handed section, a very peculiar interference figure is yielded, Fig. 345, known as Airy's spirals. These spirals are often yielded by sections of natural crystals, and are due to the twinning of right- and left-handed forms. The rotating power of a crystal decreases as the inclination of the section to the optic axis increases, until in a parallel position it is nil. The indicatrix of biaxial crys- tals is an ellipsoid, but not an ellipsoid of revolution. The two rays are both variable rays, except in the planes of sym- metry. It is in these three planes of symmetry that one ray is an ordinary ray or has a constant index of refraction and the wave front would be a circle, that of the other an ellipse. Figure 346 is a dia- gram of the indicatrix con- structed with its three axes proportional to the three indices of refraction, OX = a, OY = 3, OZ = -y. Then a ray passing through the crystal in the direc- tion of OY will be divided into two rays, one vibrating in OY with an index of refraction OY ; the other variable, and its index 198 MINERALOGY of refraction will lie between a and -y as limiting values. Similarly along the two directions OX and OZ. In general all plane sections of the indicatrix passing through O are ellipses, except in two directions when they are circular. Light passing perpendicular to all elliptical sections, as at any point p in the direction pO, will be transmitted as two plane polarized rays vibrating in planes at 90 to each other, the traces of which are the major and minor diameters of the elliptical section of the indi- catrix, cut by the plane at 90 to the direction of the entering ray. The extremities of these two diameters are the conjugate points, from which normals to the surface determine the velocity, the di- rection of transmission, and vibration planes of the two rays. The major and minor diameters of the elliptical section perpendicular to the direction of the ray are the traces of the planes of vibrations of the resulting rays in the crystal. These two diameters always bisect the angle included between the traces on the same plane, of the planes containing the ray and the optic axes, as the two planes pOA and POA'. The directions AA" and A' A"' are the optic axes, and sections of the indicatrix at right angles to these two directions are circular, as yey'e' and ycyV, and there is no double refraction, as there is no definite plane of vibration, as many or all planes are possible ; and light is transmitted along A' A'" and AA" without polarization or double refraction. Each section of a biaxial crystal, not perpendicular to an optic axis, will transmit two rays vibrating in planes and polarized in planes at 90, and when the section is revolved on the stage between cross nicols, light will be extinguished four times in 360, as in uniaxial crystals. In the orthorhombic system, where the planes of symmetry of the indicatrix are parallel to the crystallographical axes, the pina- coidal zones will show straight or parallel extinction; any other section will show symmetrical extinction as in uniaxial crystals. There are three possible positions for the plane of the optic axes, parallel to each of the three pinacoids in turn. In the monoclinic system, where one plane of symmetry of the indicatrix must coin- cide with the plane of symmetry of the system, one axis of the indicatrix will be fixed parallel to the orthoaxis of the crystal. There will be parallel extinction in one zone only, that in which the orthoaxis is the zonal axis. In all other directions there will be an extinction angle, reaching a maximum in the plane of symmetry. In the monoclinic system, the extinction angles in particular zones or on fixed planes are characteristic, particularly that of the OPTICAL PROPERTIES OF CRYSTALS 199 plane of symmetry ; and these angles are of great service in the identification of mineral species in rock sections. The plane of the optic axis when parallel to the plane of symmetry of the system is fixed, but the acute bisectrix may revolve in that plane around the orthoaxis, and the angle it makes with the vertical axis c is the measure of the angle of extinction in the plane of sym- metry and is characteristic of mineral species, but varies with the composition of the specimen. Fig. 347 is a diagram of the plane of symmetry of the amphiboles representing the extinction angles of the common varieties. The angle is measured in the clinopina- coidal section in reference to the crystalline outline or the prismatic and orthopinacoidal cleavage cracks. Again the acute bisectrix may be the orthoaxis, at right angles to the plane of symmetry, when the plane of the optic axis may revolve around the acute bisectrix as an axis; in this case the extinction angle is measured from the obtuse, bisectrix, which will lie in the plane of symmetry. In the triclinic system, any plane may be the plane of the optic axes, and there is no relation between the in- dicatrix or optical symmetry and the crystallographical axes, except in in- dividual species, where the angles of extinction are usually given in reference to some well-marked cleavage plane, or the acute bisectrix is oriented by giving the angles it forms with the normals of common crystal faces of the species. In measuring the angle of extinction, at times it is quite im- possible to determine exactly the point at which there is no double refraction or the least illumination. To the unaided eye this area may seem to extend over several degrees. At such times a sensi- tive plate is used, one by means of which the slightest double refraction may be detected. This sensitive plate is made from a cleavage piece of selenite, of such a thickness that, when mounted and slipped in the tube of the microscope in the same position as the quartz wedge, with its vibration planes at 45 to those of the nicols, will illuminate the field of the microscope evenly with FIG. 347. Diagram of the Ex- tinction Angles of the Amphi- bole. 200 MINERALOGY a red of the first order. In measuring the angle of extinction the crystal section is revolved until this even tone of red is not affected, when there will be no double refraction due to the section and the vibration planes of the section will be parallel to the planes of the nicols, and the section will be in the position of extinction. If there is the slightest double refraction, the red of the first order will change to blue or yellow, or the tone of red will be changed as the case may be, and the vibration planes of the section will not be exactly parallel to those of the nicols. Interference figures in biaxial crystals. Let Fig. 348 represent a section SS' cut perpendicular to the acute bisectrix, in which OCi FIG. 348. and O'Ci are the two optic axes. Here also, as in uniaxial crystals, light transmitted in these directions is not doubly refracted, and leaves the section vibrating in the same plane as it did on entering that of PP', the vibration plane of the polarizer, and therefore when viewed with the analyzer in position, the two points O and O' marking the position of the optic axes in the section will be dark. If the section is cut exactly perpendicular to the acute bisectrix the OPTICAL PROPERTIES OF CRYSTALS 201 FIG. 349. Interference Figure of Ara- gonite with the Plane of the Optic Axea Parallel to the Plane of one of the Nicols. two optic axes will emerge an equal distance on either side of the axis of the microscope, and the interference figure as a whole will lie symmetrically placed in the field of the microscope. The line drawn through the two points O and O' will be the trace of the plane of the optic axes on the plane of the section. The optic axes will be the axes of cones of rays which pass through the section; the in- clination or path of each will vary with the distance OO'. At some distance from O and O', depending upon the double refraction and thickness of the section and the inclination of the ray, there will emerge two rays with a phasal difference of a whole wave length. These two rays will be made to vibrate in the same plane in passing the analyzer and will interfere. The point where one ray is retarded behind the other one wave length will appear dark if monochromatic light is used, as the analyzer adds a phasal difference of 1/2 X. As the section of this cone of rays, in the plane of the mineral section, is ellipse- like, elliptical shadows or dark areas will appear around each optic axis as indicated. Alternating concentric areas of light and darkness will appear as indicated in the dia- gram, according to the phasal difference of the emerging rays. When white light is used, the concentric areas will be colored as in the interference figure of uniaxial crystals. In order to determine what portion of the field will be dark in crossed nicols, due to the light extinguished by the nicols, it is necessary to determine the 8 202 MINERALOGY direction of the vibration planes of the two rays emerging at any particular point. In the plan or upper part of the diagram, let any point whatever, asR, be taken, there will be two rays emerge, vibrating in planes at right angles. If from the point R the lines RO and RO' be drawn, they will be the traces on the plane of the section of the planes containing the ray R and the optic axes O and O'. The angle ORO' is bi- sected by the trace of the vibration planes on the plane of the section, of one ray FIG. 351. Interference Figure of Ara- emerging at R, that of the 6X- gonite with the Plane of the Optic Axes traordinary ray, as eei ; the at 45 with the Planes of the Nicols. ., , . , e vibration plane of the other ray will be at right angles to this plane, as ff . The two rays emerging at R, one vibrates parallel to ee', the other parallel to ff 7 , both have components parallel to PP' and AA', the vibration planes of the nicols, and the point R in the field will be illuminated. When all points in the field are tested, it will be found that when the plane of the optic axis is paral- lel to either vibration plane of the nicols, the dark area will be in the form of a cross, as represented in the photograph, Fig. 349. Let the stage of the ^ microscope with the section be Frr w "*~ rpvnhrorl AKO ' u j- G ' 352 ' ~~ In terference Figure of Ara- revolved 45 as m the dia- gonite with the Plane of the Optic Axes gram, Fig. 350. O, O' are revolved slightly out of the Parallel the optic axes and OO' is the Positi on, showing the Formation of the trace of plane of the optic axes, now at 45 to the planes of the nicols. The dark areas in this position will be quite different from that illustrated in Fig. 349. If any point R be taken as before, and the vibration OPTICAL PROPERTIES OF CRYSTALS 203 planes of the two emerging rays found as before, ff ' and ee' will both have components in the direction of the vibration planes of the nicols, and the point R, dark before, will now be illuminated. In the same way, if all points in the field are tested and the dark areas plotted, the dark areas would have the form of hyper- bolas, as shown in the photo- graph, Fig. 351, with the optic axes at the poles and the plane of the optic axes bisecting the curved shadow. The acute bi- sectrix is located on the con- vex side of the curved shadow midway between the two. The movement of these shadows should be Carefully FIG. 353. Interference Figure of Topaz. observed on revolving the sec- tion, as their paths and curves help, very materially, to locate the 'acute bisectrix and the direction of the plane of the optic axes when but a small portion of the interference figure is within the field of the microscope or when the section is inclined to the acute bisectrix, causing the figure to lie eccentric in the field of view. Fig. 352 is a photograph of the inter- ference figure of aragonite, re- volved just a little, showing how the cross breaks up into the two hyperbolas. Often the angle between the optic axes is so large that the optic axes emerge out of the field of the microscope ; but when the section is perpendicular to the acute bisectrix, the symmetrical, as illustrated in FIG. 354. Interference Figure of Barite, section nearly perpendicular to the Optic Axis. interference figure will still be Fig. 353. When the section is cut perpendicular to an optic axis, the curve or color areas are circles around the optic axis, as in uniaxial crys- 204 MINERALOGY tals, or possibly a little elongated at the margin, as indicated in Fig. 354, in the direction of the other optic axis. On revolving such a section the curved shadow revolves around the optic axis as a center, counter to the revolution of the section and always with its convex side toward the other optic axis. The trace of. the plane of the optic axis will pass through the pole of the curve or the optic axis, bisecting the shadow symmetrically. The optical sign of biaxial crystals. The positive or negative character of a crystal may be determined from its interference figure. The section is placed between crossed nicols, with the plane of the optic axis at 45 to the vibration planes of the nicols; the quartz wedge is then inserted, with the vi- bration plane of the slow ray of the wedge parallel to the axial plane of the section, as indicated in the diagram, Fig. 355. When the slow ray of the section vibrates in a plane parallel to the slow ray of the wedge, the circles around the optic axis will contract from the center of the figure as they disappear at the optic axes. Other color bands will contract along the long axis of the wedge, until they meet at the acute bi- sectrix, when they break, forming two circles, one around each optic axis; all the color bands will continue to contract in this manner as the wedge is advanced. The direction of this con- traction is indicated by the arrows in the diagram. The effect is that of thickening the section, and the sign of the section is the same as that of quartz, or positive (+). The heads of the arrows, indicating the direction of contraction, make a posi- tive sign with the long axis of the wedge as usually mounted. When the motion of the color bands is the reverse, or they expand from the optic axes, the section has been thinned by the advance of the wedge, and the section is the reverse of that of .the quartz, or OPTICAL PROPERTIES OF CRYSTALS 205 negative ( ). The arrows in the diagram are reversed. Care should always be taken that the same relative positions of the wedge and the axial plane exist ; for if the slow ray of the wedge is intro- duced at right angles to the plane of the optic axes, all motions of the color bands are reversed and the sign may be taken opposite to what it really is. Also in sections of negative crystals after the point of compensation has been reached and the wedge is still ad- vanced, the effect is as if thickening the section, or a positive crystal. Measurement of the angle between the optic axes. I. The ap- proximate value of 2 E may be obtained by measuring the distance between the two poles of the hyperbolas in the interference figure of a section perpendicular to the acute bisectrix and in the 45 position with a micrometer eyepiece. Placing this value at 2 d, then sin E = d/C, where C is a constant for the combination of lenses used, and may be determined by a section in which the angle between the optic axes is known, as aragonite. II. After the three indices of refraction have been determined, the angle 2 V may be calculated from the formula, cosV = As the value of 2 V is influenced considerably by variations in the fourth decimal place of the value of the indices of refraction, this method is not as accurate as the direct determinations of the angle. III. In the direct determination a section of the crystal is required, cut perpendicular to the acute bisectrix. The section is mounted in the axial angle goniometer, with the plane of the optic axis at 45 to the vibration planes of the nicols. One hyperbola is brought tangent to the hair of the eyepiece and a reading taken ; then the second hyperbola is brought tangent to the hair and a sec- ond reading taken ; the difference between the two readings is the value of 2 E, the axial angle measured in air ; 2 V, the true axial angle, may be calculated when the median index of refraction p is known, sini smE sin r sin V 2 E is always greater than 2 V ; and when 2 V is large, the ray along the optic axes is often totally reflected at the surface of the section, 206 MINERALOGY and the angle in air would be 180. It is then necessary to immerse the section in a strongly refracting liquid, which decreases the apparent angle. 2 H is the term used when measured in a strongly refracting liquid, as oil or any of those liquids given under the determination of the index of re- fraction, page 215. Then sinV = sin H, where n is the index of re- P fraction of the liquid in which the section is immersed. Dispersion of the optic axes. When light of different wave lengths is used in the measurement of the axial angle, the value will differ with different colors and change progressively from one end of the spectrum to the other. This change of the axial angle for light of different wave lengths is termed the dispersion of the optic axes, Fig. 357. Whether the angle is greater for red light, p, than for violet light, v, or the reverse, will depend upon the relative values of the three indices of re- fraction for these individual wave lengths. When the angle is greater for violet light than for red, it is ex- pressed u>p, and the reverse, FIG. 356. This can usually be deter- mined by a close inspection of the interference figure yielded by white light. If the angles for all colors were the same, i.e. no dispersion of the optic axes, the hyper- bolas for each color would lie in the same position and FIG. 357. Dispersion of the Optic Axes. those for all colors would be superimposed; but when they differ for different wave lengths, as when that for red is greater than that for violet, the hy- perbola for red is farther away from the acute bisectrix than that for violet ; and when white light is used, the red wave will be OPTICAL PROPERTIES OF CRYSTALS 207 suppressed along the concave side of the hyperbolas, the side far- thest away from the acute bisectrix, and the color appearing will be white light minus red, or blue. On the convex side of the hyper- bolas will be white light minus violet, or red. The convex side is red when p>u, and the convex side is blue when v > p. In the orthorhombic system the dispersion of the optic axes takes place in one of the planes of symmetry of the crystal, and the acute bisectrix holds the same position for light of all colors, except when 2 E is nearly 90. 2 E for red may be less than 90 with the vertical axis c the acute bisectrix, and greater than 90 for violet when one of the lateral axes would be the acute bisectrix for the violet wave. Again the plane of the optic axis may change from one pinacoidal plane to another with the wave length, as is the case in brookite, in which the plane of the optic axes for waves including red to yellow is parallel to the base, with the brachyaxis as the acute bisectrix ; for waves shorter than yellow the plane of the optic axis is parallel to oio, or macro- pinacoid, with the brachyaxis still the acute bisectrix. In such a case the plane of the optic axis is said to be crossed, or the dis- persion is crossed, and there must be a wave length of light a little shorter than yellow for which the angle between the optic axes is 90, or for which the mineral brookite would be uni- axial. Dispersion in the mono- clinic system. In the monoclinic system, where but one of the axes of the indicatrix is fixed in rela- tion to the crystallograph- ical axes, that in the di- rection of the orthoaxis, three kinds of dispersion are possible, as the three axes of the indicatrix, a, p, y, each in turn may be parallel to the orthoaxis. 1. When p is parallel to the orthoaxis, the plane of the optic axes will lie in the plane of symmetry of the crystal and will be fixed at 90 to the orthoaxis, but may revolve around it as an axis. FIG. 358. Inclined Dispersion of the Optic Axes. 208 MINERALOGY It is not necessary that the angle between the acute bisectrix and the vertical axis should be constant, and it is constant only for individual species when chemically pure. The value of this angle will change with the composition of the mineral, and with the wave length of light for the same composition. In common hornblende this angle is 19 q 53' in the obtuse angle p. Expressed, Bx aA C = 19 53' in front; this is also a measure of the extinction angle, which is inclined. If monochromatic light of different wave lengths is used, it will be found that this angle will vary with the color of light used. The interference figure as a whole is displaced, and that of one. color will not be superimposed on that of another, yet the trace of the plane of the optic axes will divide them all symmetrically. This is termed inclined dis- persion, Fig. 358. 2. When the ortho- axis is the obtuse bi- sectrix, the plane of the optic axis may ro- tate around it as an axis of revolution; and the interference figure for each wave length of light which is in the section perpendic- ular to the acute bi- sectrix may be dis- placed sidewise through an arc measured in the plane of symmetry. The trace of the plane of the optic axes will also be displaced through this same angle for each wave length. The traces of the planes for each color will lie parallel on the section, but the planes will all intersect in the obtuse bisectrix or orthoaxis, which is fixed, Fig. 359. This is termed horizontal dispersion. 3. When the acute bisectrix coincides with the orthoaxis. Now the interference figure will lie in the clinopinacoidal section, and will revolve around the acute bisectrix as a center. The traces of the plane of the optic axes for light of the various colors will all pass through the fixed point, the center or acute bisec- trix, Fig. 360. This is termed crossed dispersion. Dispersion in the triclinic system. In the triclinic system, FIG. 359. Horizontal Dispersion. OPTICAL PROPERTIES OF CRYSTALS 209 FIG. 360. Crossed Dispersion. where there is no fixed direction to which any of the axes of the indicatrix must conform, it is possible for all varieties of disper- sion described in the other systems to take place at one and the same time, and the interfer- ence figure may be entirely without symmetry. Methods of determining the indices of refraction. The index of refraction of any substance is a physical constant, characteristic of the substance. It not only serves as a means of identi- fication, but also as a meas- ure of purity. The value of the index of refraction varies with the temperature, but this variation in case of solids, at ordinary temper- atures, is small and within the limits of error, therefore negligible. The index of refraction, when all precautions and when great care are taken, together with an average of several observations, may be determined within .0002. The value of the index of refraction for minerals will lie between 1.3, that of ice, and 3.08, that of pyrar- gyrite. I. The most ac- curate method is that in which the angle of deviation of the re- fracted ray is ac- tually measured, as transmitted through a prism of 60, or one not varying more than 5 from 60. As the angle of deviation will vary with the inclination of the ray, the angle of least deviation is found and measured as follows. In Fig. 361 abc is the prism, with the angle at a nearly 60. This angle is accurately measured with the goniometer. The ray of FIG. 361. 210 MIXKRALOGY light RO enters at O, is refracted to O', and is again transmitted, on leaving the prism, in the direction of O'R'. The angle of deviation due to the prism is R'de = R'O'e' = 8. This will be at a mini- mum, or is the least deviated, when the ray passes through the prism symmetrically as drawn in the figure. sin * ; i = ROn. The angle bac = a; dab = 1/2 a and axO sin r 90 aOl = lO'a = 90. VxOl = xaO = nOP = 1/2 a = r, the angle of refraction. Draw O V parallel to Re, then from the symmetry of the figure the angle e'O'R' = R'de = B = e'O'P + P'O'R', POR = dOx = e'O'P'; also dox = P'O'R' = POR = 1/2 8, but NOP = xOl = 1/2 a. i = NOR = 1/2 a ' + 1/2 8 and n = ^J = sin 1/2 (a + 8) ? where sin r sin 1/2 a the angle a is carefully measured with the goniometer. The angle of least deviation is found as follows : The telescope of the goniometer is set exactly opposite the collimator and the direct ray through the Websky slit is observed and adjusted to the cross hairs, when a reading is taken with everything clamped. The graduated circle and telescope remaining clamped, the crystal is mounted and adjusted so that the edge at the angle a is parallel to the vertical hair. It is then pushed in with the screw, between the collimator and the telescope, until the image of the slit disappears on looking in the telescope, the base of the prism being to the left. With 'the graduated circle still clamped, the telescope is now undamped and revolved to the left until the image or signal reappears, then the prism is revolved back and forth through a small arc, at the same time following the signal with the vertical hair of the telescope. It will soon be seen that on revolv- ing the prism there is a maximum position to the right for the sig- nal, and having reached this position, even though the prism is still revolved the same way, the signal moves up to this position then reverses its motion or turns back. This maximum position marks the point where the ray is symmetrical to the prism as drawn in the diagram, Fig. 361. By moving both the telescope and prism at the same time, the vertical hair is brought to this maximum posi- tion of the signal and a reading taken. The difference between the original position of the signal and this of the least deviation will be the angle 8. When white light is used, there will be a series of colored signals, one for each color, due to dispersion, but only one OPTICAL PROPERTIES OF CRYSTALS 211 image when monochromatic light is used. Having measured the angle of least deviation, the index of refraction is obtained from the formula above. In all isotropic substances n will be of the same value, whatever the relation of the edge of the prism to the crystal may be. This is not the case in anisotropic substances, where the prism edge must be cut with a definite relation to the axes of the indicatrix. In the tetragonal and hexagonal systems, the edge of the prism containing the angle a should be cut parallel to the base, and the plane bisecting the angle parallel to the vertical axis, or the prism edge containing a may be cut parallel to the vertical axis. In either case, in measuring the angle of least deviation, two signals will appear, one caused by the ordinary ray, the other by the extraordi- nary ray. These two readings substituted in the formula will yield two indices of refraction, one that of the ordinary ray co, the other that of the extraordinary ray . In biaxial crystals, where there are three indices of refraction to be determined, two prisms are necessary. One must be cut with the edge containing the angle a parallel to an axis of the in- dicatrix, and the plane of symmetry of the indicatrix containing this edge of the prism must also bisect the angle a. The second prism must be cut in the same relation to a second axis and plane of the indicatrix. Each of these prisms will yield two signals as in the uniaxial prism and therefore two indices of refraction. One prism will yield a and p, the other p and -y- The index repeated or determined in both prisms will depend upon the axes of the in- dicatrix to which the edges of the prisms are parallel. II. Method of total reflection. Fig. 362 is a diagrammatic section of the Abbe Total Reflectometer, in which C is a hemisphere of Jena flint glass, having an index of refraction n = 1.8904, the upper surface of which, b, is polished to a true plane passing through the center of the sphere and adjusted so as to pass through the axis of the vertical graduated circle from which the readings are taken and which is not represented in the diagram. A polished section S is cut parallel to a plane of symmetry of the indicatrix. Often a cleavage surface will fill this requirement. A small drop of a highly refracting liquid, usually methylene iodide, n = 1.742, having first been placed on the center of the plane b, then the pol- ished face of the section S is placed gently on the hemisphere with a thin film of the highly refracting liquid separating the two sur- faces. M is a mirror which reflects light in the required direction. 212 MINERALOGY The limiting ray tO, striking the surface of the section at O, is totally reflected in the direction Ot'. The angle t'Oe is the critical angle. Any ray, as mO, within this angle will at the point O be re- FIG. 362. fracted and pass out in the direction Ot", and some will be reflected within the angle t'Oe, and the field t'Oe will be semi-illuminated. Any ray, as 8O, striking the plane surface s at an angle greater than the critical angle eOt', will be totally reflected in the di- rection of O8' and the field a'Ot' will be fully illuminated. At the boundary between these two fields, t'O will be marked by a shadow. The field of the tele- scope, represented by the circle F, is brought with the cross hairs to the shadow, and a reading which gives the critical angle eOt' is taken. Then n = N sin r, where N is the index of refraction of the glass hemi- sphere and r is the critical angle. This method has the advantage of the possibility of determining all three indices of refraction in one and the same "section. As OPTICAL PROPERTIES OF CRYSTALS 213 one ray is constant, the shadow caused by it will not change on completely rotating the hemisphere with the section. The other ray will increase from a minimum to a maximum according to the position of the section; and these two limiting values will represent the critical angles for the other two indices of refraction. The telescope is fitted with a nicol so that light vibrating only in the plane required may pass and illuminate the field. The ray may also be adjusted to enter the section as represented in Fig. 363, and the field a'Ot' will be entirely dark, and the field t'Oe will be illuminated and the contrast between the two fields will be greater. III. A convenient refractometer, as constructed by Herbert Smith of the British Museum and illustrated in Fig. 364, is so ar- FIG. 364. Refractometer. ranged that the index of refraction may be read directly from a scale in the instrument to the second decimal place and the third estimated. The specimen is placed on the highly refracting glass with a film of liquid between, as in the Abbe instrument. The light entering at O, the shadow is thrown on the scale and the reading taken. In double refracting substances two shadows will be noted, indicating the two indices of refraction ; the section is revolved until these are maximum or minimum values. This instrument is very convenient for jewelers in the determination of the refraction and identification of cut stones. IV. Cleavage pieces of transparent minerals may be used to determine the index of refraction with the microscope. The thick- ness of the section S, Fig. 365, is measured with a micrometer cali- 214 MINERALOGY pers = T. A slide with a fine scratch is placed on the stage and the microscope very carefully focused on the scratch o. Then the mineral section is placed over the scratch. The scratch will now not be in focus, but will again come in focus by turning the fine adjustment of the microscope, so as to lift the objective; o will appear as if at o'; the distance oo' which the scratch seems to have been lifted will depend upon ^ the thickness of the section and ^/ FIG 365 the index of refraction of the min- eral. The apparent thickness of the section do' is determined by measuring the distance oo' with the micrometer of the microscope, and subtracting oo' from T. Then,n = or The actual thickness , for n = sin i tan i sin r tan r T oo' The apparent thickness - f , as for these small angles the ratio of the tangents is practically the same as that of the sines. This method is only an approximate one and thick sections must be used. Ordinary rock sections are too thin for accuracy. V. A convenient method of determining the index of refraction of minerals in powder or very small crystals is to mount them on a slide in fluids of different indices of refraction. A list of such fluids FIG. 366. Section of a Quartzite, showing the Low Relief. Crossed Nicols. which cover the range of the indices of refraction of most important rock-forming minerals, as given by P. E. Wright, are :- OPTICAL PROPERTIES OF CRYSTALS 215 1.450-1.475 . . Mixtures of petroleum and turpentine. 1.480-1.535 . . Turpentine and ethylene bromide. 1.540-1.635 . . Clove oil and -monobromnaphthalene and a-monochlornaphthalene. 1.660-1.740 . . a-monobromnaphthalene and methylene iodide. 1.745-1.790 . . Methylene iodide and sulphur. When the index of refraction of the crystal fragments are the same as the fluid in which they are mounted, the outlines of the fragments are very indistinct and scarcely visible. This is well illustrated by the glass rod in the Canada balsam bottle, the indices of which are nearly the same, and the rod is scarcely visible. Quartz mounted in balsam is also indistinct, as the indices co = 1.544 and = 1.553, while for balsam n = 1.548, just between. In a section of quartzite where the grains are all the same color, their outlines will be invisible in ordinary light ; but in crossed nic- ols, where they will be differently illuminated, they are quite distinct, Fig. 366. The outlines of mineral fragments are more marked or distinct, the greater the difference between the indices of refraction of the two substances. In calcite, where = 1.486 and co = 1.658, mounted in balsam the outlines are distinct and the surface of the section is rough, Fig. 367. It is said to have a marked or high relief. With the polarizer only in, if the section is revolved through 90, the relief for the two rays will be noted to be quite different, as the difference in one case is .11 and in the other .062. The relief then is a measure of the difference of the index of refraction of the mineral and the index of refraction of the medium in which the section is mounted. VI. Becke's method, or the determination of the relative values of the indices of refraction of two adjacent minerals in the same section. Where the index of refraction of one is known, that FIG. 367. Section of Calcite showing Marked Relief and Rhombohedral Cleavage. 216 MINERALOGY of the other may be approximately estimated . This method depends upon the principle that light passing from a rarer medium or one with a lower index of refraction to one with a higher index of re- fraction will pass at all angles and there will be no critical angle ; but in the reverse direction there will be a critical angle and some of the light is reflected to the side of the higher index of refraction. The illumination on the side of the higher index of refraction will be brighter, as some of the rays are totally reflected to that side. As an example the index of refraction of quartz and orthoclase in a section of granite may be tested in this way. The effect is best seen with a medium high objective and with a converging light, the light being diaphragmed off so as to illuminate only a small area. If the microscope is carefully focused on the boundary between the two minerals, then the objective slowly lifted, a line of light parallel to the line of contact will appear to move in the direc- tion of the mineral with the higher index of refraction, or on the side of quartz; the difference in this case on the average is .02. With careful work and experience a difference of .001 may be detected by this method. Study of minerals in rock sections. In the study of a mineral section or of minerals in rock sections, an order of observation should always be followed. The section should be carefully studied in all parts with the low power adjective in order to de- termine the relative abundance of each mineral species, their rela- tions and relative size, and favorable sections should be chosen for the optical observations, then the following order should be followed : 1 . Color. When the mineral is opaque, it is observed in reflected light, with the mirror under the stage turned off. Minerals which are opaque in thin sections are of metallic luster, as magnetite, pyrite, pyrrhotite, or chromite. Transparent sections are observed in transmitted light and the color noted ; whether evenly colored or irregular, caused by a difference in chemical composition or due to inclusions, cavities, etc. 2. Form. The outline of each individual species is noted if bounded by straight lines, i.e. crystal faces well developed, the individuals being euhedral or idiomorphic, in contrast to those irregular in outline, which have no well-defined crystal faces developed and are anhedral or allotriomorphic ; such irregular in- dividuals usually act as a matrix, filling the cavities between the minerals of earlier crystallization, which are often of larger size, with distinct outlines or phenocrysts. OPTICAL PROPERTIES OF CRYSTALS 217 3. Cleavage cracks, partings, and fractures due to pressure or strain should be noted and the angles between two well-defined cleavage directions, where they occur, measured in a number of sections, in order to obtain the true angle or that measured at 90 to the intersection of the two cleavage planes. 4. Note the crystal outlines where well developed and where elongated ; the direction of the elongation in regard to the crys- tallographic axes is determined, also any irregularity of the outline, as that due to corrosion, resolution, alteration, or weathering, and whether these changes are restricted to the surface or have fol- lowed cleavage cracks and fractures ; also the nature of the alter- ation product is noted. 5. Index of refraction. Rock sections are usually mounted in Canada balsam, the index of refraction of which is approximately 1.539, but varies slightly with the amount of solvent it contains or with the age of the mounted section, as the balsam is constantly hardening with age. Specimens with an index of refraction near that of balsam will have little or no relief ; their surfaces will appear flat and smooth. It is well to have several mineral sections mounted for comparison, the index of each being known ; their relief may be compared with the unknown section and the index of refraction approximately determined. Sodalite, 1.483; leucite, 1.508; orthoclase, 1.523; quartz, 1.547; beryl, 1.584; olivine, 1.670; and rutile, 1.712, are good minerals for comparison. A specimen with an index of refraction above 1.60 or below 1.50 will have a rather high relief. The cracks, as cleavage, scratches on the surface made in grinding the section, and the outline, all will appear well marked and distinct. Whether the refraction is above or below 1 .549, that of balsam, can be determined by Becke's method. Minerals with a high relief seem particularly rough when the section is shaded from reflected light by passing the hand up and down in front of the microscope. 6. In crossed nicols and parallel light. If the section remains dark between crossed nicols when the stage is revolved, it is either amorphous, as glass, isometric, or a double refracting mineral cut perpendicular to an optic axis. An anisotropic section between crossed nicols will yield an interference color which will be a measure of its double refraction and the section will extinguish every 90, that is, each time the vibration planes of the section are parallel to the cross hairs of the eyepiece. 7. The angle of extinction is measured in reference to the vertical 218 MINERALOGY axis, as determined in the section by the cleavage, outline, or elon- gation. Whether it is parallel or inclined is noted and the angle of extinction measured, also the twinning of the section may be seen in relation to the extinction angles. 8. Pleochroism. This is caused by the light being absorbed along one plane of vibration in the section differently from that along the other. The section is turned on the microscopic stage until extinction, when the vibration planes of the section will be parallel to those of the nicols. The analyzer is pushed out of the line of view, when the section will be illuminated by the rays vibrating parallel to the one plane of vibration in the section only, that of the polarizer, usually running from right to left. The color of the section and the degree of illumination are both noted, as is also the relief of the section ; then the section is revolved on the stage 90, when the light will be vibrating parallel to the second plane of vi- bration of the section, and any change in color, shade, or relief is noted. 9. In crossed nicols and converging light. All sections of the mineral under observation in the specimen are carefully examined, and one selected as nearly perpendicular as possible to the optic axis, in uniaxial crystals, and to the acute bisectrix, in biaxial crys- tals, is chosen. The interference figure is observed and the uniaxial or biaxial character of the crystal noted, as well as the approximate axial angle in the latter. 10. When uniaxial, the optic sign is determined with the mica plate ; and when biaxial, with the quartz wedge. Where the inter- ference figure is well formed, the character of the dispersion may also be noted. PART II CHAPTER I THE RELATION OF MINERALS TO THE ELEMENTS MINERALS, when considered from a chemical standpoint, are either elements in the uncombined state or are combinations of elements which have been brought together during the past ages and united by chemical forces which differ in no way from the same forces with which we have become acquainted in the labora- tory. Some mineral molecules are simple combinations of a few chemical elements ; and the same compounds are daily produced in the laboratory, in the simple processes of chemical analyses, as in the precipitation of calcium carbonate from a solution of a soluble calcium salt with an alkali carbonate. If this is carried out at room temperatures and the precipitate is allowed to stand, it will crystallize, forming calcite. If the same precipitation is carried out at temperatures near the boiling point of water, that is, on the water bath, and allowed to crystallize in a hot solution, the form aragonite will be produced. Here are two minerals, calcite and ara- gonite, one crystallizing in the hexagonal system, the other in the orthorhombic system. They are both calcium carbonate, and chemically they are identical ; but physically they are different, and as in the precipitation they are formed under different conditions, they are two phases of the same chemical substance; moreover, calcium carbonate directly after a rapid precipitation is amorphous, another phase, and becomes crystalline only after standing. This property of chemical compounds, of occurring in different physical forms is known as polymorphism. Calcium carbonate is thus tri- morphic. It may be amorphous, it may possess the molecule of calcite, or it may possess the molecule of aragonite. Silica, SiO 2 , is said to possess six different forms or phases. The various forms of a polymorphic substance will never under the same conditions possess the same degree of equilibrium, and one form will be more stable than the others. It is always the 219 220 MINERALOGY tendency of one form, the less stable, to pass over to the other, the more stable form, under the fixed conditions. Calcium carbonate when quickly precipitated from solution separates as an amorphous solid, and on standing passes over to the more stable crystalline forms, calcite or aragonite, according to the temperature of the solution, as calcite is the more stable form in cold solutions and aragonite is the more stable form in hot solutions. When the change of phase proceeds in one way only, the compound is said to be mono- tropic, but if the change may go back and forth with the change of temperature, or is reversible, the compounds or phases are said to be enantiotropic. Such a case is represented by sulphur. Sulphur when fused and then allowed to solidify at a temperature above 96 will form monoclinic crystals, which are the stable phase between 96 and 120, the fusing point of sulphur. Below 96 these mono- clinic crystals become brittle and clouded and pass over to ortho- rhombic sulphur, a phase more stable than the monoclinic form at low temperatures. If the temperature is again raised above 96, the reverse of this will take place and in time the orthorhombic sulphur will form monoclinic crystals. The orthorhombic phase is the more stable below 96, and the monoclinic phase is the more stable above 96. There are several other phases of sulphur, but at ordi- nary temperatures these are all less stable than the orthorhombic phase, and it is for this reason that all natural occurring sulphur is of the orthorhombic phase. Graphite and diamond are different phases of the same element, carbon. There are numerous other examples of dimorphism in minerals, as sphalerite and wurtzite; quartz and tridymite ; smaltite and safflorite ; pyrite and marcasite. Pyroxenes and amphiboles are probably dimorphous conditions or phases of the same compound, and even trimorphic cases occur, as in the three minerals rutile, anatase, and brookite, all three of which are different phases of TiO 2 . Source of the elements, Minerals are the source of all the ele- ments ; it has been through the chemical study and the analyses of minerals that, with few exceptions, all the elements have been dis- covered. Those elements which occur in nature in the uncombined state as minerals are few in number and are restricted principally to the group known as metals ; as platinum and the platinum group, silver, mercury, gold, copper, lead, bismuth, iron, arsenic, and anti- mony. Some of these, as iron, lead, antimony, arsenic, and bis- muth, are rare as native elements, though quite common enough as constitutents of minerals when combined with other elements. THE RELATION OF MINERALS TO THE ELEMENTS 221 The non-metallic elements sulphur and carbon occur in the uncom- bined state in nature in considerable quantities. Of the eighty established elements, oxygen far surpasses any other in its wide distribution, forming one fifth of the atmos- phere, eight ninths of the water, and from 45 to 50 per cent, of the earth's crust. The results of careful calculations indicate that the amount of oxygen is hardly equaled by all the other ele- ments taken together, or oxygen forms about 50 per cent, of the earth as known by man. Oxygen enters the composition of a large num- ber of minerals as an important factor. Of the other elements there are seven, silicon, aluminium, iron, calcium, magnesium, sodium, and potassium, in their order of abundance, each of which composes at least 2 per cent, of the earth's crust, and they are universally dis- tributed. The above eight elements compose at least 97 per cent, of the earth as known to man. Metals to which we have become accustomed, through their use in our daily life, thinking of them as common elements, as copper, lead, zinc, silver, or gold, occur only in restricted localities; and owing to their commercial value the minerals containing them have been mined, with a constant accumulation of the metal reduced. Elements such as titanium, thorium, cerium, tungsten, uranium, and molybdenum, which even the chemist, in the past, considered very rare, are at the present time becoming well known in the commercial world, from recently discovered uses to which their properties adapt them. These rare elements are important constituents of but comparatively few minerals, and these are usually restricted to localities where many of them occur associated together. Such localities are constantly being searched for by the pros- pector, urged on by the constant demand and increase in price, as several of these rare elements have been proven useful in the production of special steels, in the manufacture of lighting man- tles, and in incandescent lamp filaments. At the present time salts of some rare earths are being produced by the ton as by-products, which had chemists wished to secure in pound lots only, it would a few years ago have been impossible. Of the most important elements the following list includes those which are found in the earth's crust in amounts exceeding .02 per cent. : Oxygen 49.98 Sodium 2.28 Phosphorus .09 Silicon 27.21 Potassium 2.23 Manganese .07 222 MINERALOGY Aluminium 7.26 Hydrogen .94 Barium .05 Iron 5.08 Titanium .41 Sulphur .04 Calcium 3.51 Carbon .22 Nitrogen .02 Magnesium 2.50 Chlorine .15 Strontium .02 Chromium, fluorine, lithium, and uranium are all less than .02 and probably greater than 0.01 ; bromine and all other elements are each, in turn, less than .01. In a classification of the minerals their most important charac- teristics must be considered, and the starting point of a natural- classification is without doubt one in which the chemical composi- tion and physical properties are the most important consideration, though the latter to a large extent are derived from the elements a mineral contains. Those minerals which are composed largely of the same elements should stand in any scheme of classification near together, especially since mineral species are mostly deter- mined by the chemical tests for the elements which they contain. The classification of minerals will follow closely the natural classi- fication of the elements themselves. It has been shown, especially by Mendeleef, that the properties of the elements are determined by their atomic weights, and from a consideration of this fact a natural classification of the elements has been adopted which places the elements in the order of their atomic weights. In a list of the elements placed in the order of their atomic weights, starting with lithium, the first element which possesses a well-developed chemical character, the atomic weight increases un- til sodium is reached, with an atomic weight of 23. Sodium is an element very similar to lithium in its chemical properties and very different from fluorine, with an atomic weight of 19, directly pre- ceding it. Sodium is then written in the column with lithium and a second horizontal line or group is started. In each case, elements having similar or related chemical prop- erties will fall in the same column, and these properties will be repeated periodically. This whole scheme is known as the periodic classification of the elements. Elements falling under each other in the same column of the table of elements are of the same valence and are capable of replacing each other in mineral molecules to a large extent, though this prop- erty may not extend from top to bottom of the column in all groups. In the first column or group, lithium, potassium, rubid- ium, and caesium fall directly under each other. These are all alkali THE RELATION OF MINERALS TO THE ELEMENTS 223 3* 10 || II 1! is- i a .s 111 Hi 111 .2 05 73 CO oo 00 |8 M C5 ^ II 03 CD s till & Phosphorus P= 31.04 Vanadium V = 51.0 Arsenic As - 75.0 Columbium Cb = 93.5 Antimony Sb = 120.2 00 01 oo Tantalum Ta= 181,5 ' r 02 O C3.2 00 02 Is 00 II CO ^ PH OJ t> Sob 00 ir|3 co I II ^M OO 02 II 3 K gu SO 2 , the salt is said to be normal, or, as rep- THE RELATION OF MINERALS TO THE ELEMENTS 227 resented by the formula, a normal sulphate. When only one of the hydrogen atoms is replaced, jj Z Q / ^ 2 > an ac ^ sa ^ * s * ori ned, acid sodium sulphate. 2, normal magnesium sulphate ; or again, only a part of the hydroxyl may be replaced, iQH/^ 4 ' ^ as ^ c ma S nesmm sulphate. Malachite is a basic copper carbonate, -u _ Q __ Q U /COs, in which one bond of the copper is taken up by hydroxyl and the other by the acid radicle. In each group of minerals, as the sul- phates, there are possible normal anhydrous salts ; normal salts with water of crystallization, or hydrous salts ; basic salts ; and acid salts ; and in some of the complex mineral molecules there may be hydrated and acid and basic water present. It is not always possible to determine the structural formula of a mineral, or to tell just what the exact relations of each atom are. The empirical or percentage formula is calculated from the results of analysis, as this is simply the reverse of the calculation of the percentage of any element from a given formula. The percentage of each oxide is divided by its molecular weight, which will give the ratios of the various oxides in the formula; thus in case of the sulphate of calcium, the mineral gypsum : RATIO CaO = 32.5 -s- 56.1 = .579 = 1 SO 3 = 46.6 *- 80.6 = .577 = 1 H 2 O = 20.9 * 18.0 = 1.15 = 2 Dividing the ratios by the least of their number, as there must be whole atoms in the molecule, we obtain for CaO, one ; for SO 3 , also one ; and for H 2 O, two. The percentage formula will be CaO, SO 3 2H 2 O, or CaSO 4 , 2 H 2 O. In the mineral and crystalline molecule cer- tain groups of elements have been found able to replace each other, and at the same time the form or angle of the crystals will be but slightly changed. Such groups of elements are very closely related to each other in several ways. Their molecular volumes are very 228 MINERALOGY nearly alike. Their crystals when pure are nearly of the same angle. Compounds which replace each other are usually alike chemically in that they are salts of the same acid, as carbonates, sulphates, or phosphates ; but this is not always true, since sodium nitrate, NaNO 3 , crystallizes in the same forms and nearly the same angles as calcite, CaCO 3 . Elements or compounds which replace each other in the crystalline molecule in all proportions are said to be isomorphous. Formerly they were thought to be of exactly the same form and angles, but at the present time it is thought that it is only necessary for the angles and molecular volume to be of nearly the same value. This is well illustrated in the isomor- phous group of natural carbonates: TAT MOLECULAR VOLUME Calcite CaCO 3 74 55' 36.8 Magnesite MgCO 3 72 36' 27.8 Siderite FeCO 3 73 00' 30.3 Rhodochrosite MnCO 3 73 00' 31.9 Smithsonite ZnCO 3 72 20' 28.0 The molecular volume or volume of the unit of the space lattice is found by dividing the molecular weight by the specific gravity of the substance. If the molecular weights of crystalline sub- stances were the same and they differed in specific gravity, then the same volume of the denser substance would contain more molecules per volume than the less dense substance, and the molecular vol- ume or the relative size of the space-lattice units will vary inversely as the specific gravity. Topic parameters represent the relative distances or the ratios of the distances between centers of the simple structural units of the space- point-system, measured along the three axial directions. The topic parameters are functions of the molecular volumes and the axial ratio of a compound. The crystalline angles will vary directly with the composition; in pure calcite, with a rhombohedral angle of 74 55', as an end mem- ber of a series in which pure smithsonite, with a rhombohedral angle of 72 "20', is the other end member, every molecule of zinc carbonate crystallizing with the calcium carbonate will have the effect of proportionally decreasing the angles of the calcium carbonate. The amount of decrease in the angles will be directly proportional to the percentage of zinc carbonate present. If there is no car- bonate present, other than zinc carbonate and calcium carbonate, in the crystal, their percentage proportion could be calculated from THE RELATION OF MINERALS TO THE ELEMENTS 229 a measurement of the angles. The angles are then a function of the chemical composition. Mesitite is a naturally occurring carbonate, forming crystals in which iron and magnesium carbonates have crystallized in pro- portions of two of magnesium to one of iron, and they should each have the same proportional influence on the crystalline angles : r A r Mesitite, 2 MgC0 3 , FeC0 3 2 MgCO 3 = 145 12' FeCO 3 = 73 218 12' -f- 3 = 72 46'; the measured angle is 72 46'. The increase or decrease of the topic parameters with the addi- tion of isomorphous substances in the molecule may not be the same in all directions. These parameters of mixed crystalline substances will increase more rapidly in one direction with the increase of the percentage of certain elements than in others. This influence will have more effect upon molecules which are compara- tively simple in their structure than on those which are complex ; and compounds which are isomorphous in complex mineral mole- cules may not be able to replace each other in such simple mole- cules as the chlorides. Thus potassium and sodium are isomor- phous in many silicates, yet their simple chlorides crystallize in different types of symmetry. Owing to the unequal increase in the topic parameters, complex isomorphous groups, as the pyroxenes and amphiboles, may pass through three entire cystallographical systems. Generally the physical properties of mixed crystals, or those formed by isomorphous groups replacing each other, will stand as a mean between the properties of their constituents ; and, in the strict sense of the term, compounds are isomorphous when the physical properties of their mixed crystals are continuous functions of their chemical composition. Elements which stand in the same column in Mendeleef s table of the elements, or those of the same group, are usually isomor- phous, especially in complex mineral molecules, and those which fall directly under each other in the odd and even groups are iso- morphous to a great extent in simple molecules. In group I, the alkali or univalent metals, it will be noted that lithium is written on the left, while sodium is written on the right and not directly under lithium. Two columns of elements are thus formed ; any metal, as potassium, is more closely related to the 230 MINERALOGY metals of the same column, as lithium and rubidium, than it is to the metals of the other column, as sodium. The elements lithium, potassium, rubidium, and caesium are isomorphous in such simple salts as the chlorides, nitrates, iodides : or sulphates; but sodium is isomorphous with these only in more complex compounds, as the feldspars, pyroxenes, or amphiboles, and complex silicates gener- ally. Copper, silver, and gold are isomorphous in their sulphides and as elements. II. In the bivalent metals there are two distinct groups ; the first, calcium, strontium, barium, and lead, are isomorphous in their carbonates, sulphates, silicates, and practically in all minerals. The second bivalent group is composed of calcium, magnesium, manganese, ferrous iron, nickel, cobalt, zinc, and cadmium. Thek oxides are isomorphous in the spinel group, carbonates, arsenates, tungstates, silicates. Like sodium in the univalent groups, cal- cium is a connecting element in the bivalent group ; it is a member of both, forming a carbonate which is rhombohedral, crystallizing with the second group, and a second carbonate which is ortho- rhombic, aragonite, crystallizing with the first group. The two groups can replace each other to some extent in the complex silicates, as the pyroxenes and amphiboles. III. The trivalent elements, with the exception of aluminium and ferric iron, are not common; these "two replace each other in such simple molecules as the spinels ; Cr 2 O 3 is also included here. They are found replacing each other throughout the silicates, where Mn 2 O 3 and Ti 2 3 may be added, as in the garnets. IV. In the fourth group, TiO 2 , Sn0 2 , ZrO 2 , SiO 2 , and ThO 2 are found replacing each other in the rutile-cassiterite group of the tetragonal system. In silicates ZrO 2 , TiO 2 , and SiO 2 are found the more often replacing each other. V. In the group of pentoxides, phosphorus, arsenic, and vana- dium replace each other, as in the apatite group, and added to these are antimony and bismuth, which are all isomorphous in their sulphides and thiosulphates. VI. In the sixth group, sulphur, selenium, and tellurium are iso- morphous in the bivalent state only. In the sexvalent state the sulphates, molybdates, chromates, and tungstates are very closely related. VII. In the fluorine group, fluorine itself stands apart from the other members of the group, the three most important of which THE RELATION OF MINERALS TO THE ELEMENTS 231 are chlorine, iodine, and bromine, which are completely isomor- phous in such simple salts as those of silver. Fluorine enters as an isomorphous element with them only when the molecule becomes complex, as in the silicates, when hydroxyl (OH) may also replace them, as in topaz. From a consideration of the above isomorphous groups of ele- ments which may replace each other in the simple mineral mole- cule, not only will the number of elements in each isomorphous group increase with the complexity of the mineral molecules, but in the more complex silicates whole groups of elements replace each other, In the amphi boles such groups as Na 2 , H 2 , (A1 2 OF 2 ), (Fe 2 OF 2 ), (A1 2 O(OH 2 ) 2 ), (Fe 2 O(OH 2 ) 2 ) are considered to be isomor- phous. It is readily appreciated that mineral species are with rare exceptions never pure chemical compounds, constant in their percentage composition, where such replacements are possible. In the attempt to deduce from the percentage analysis of any min- eral its formula, and thus its relation to other mineral species, it is always necessary to group the equivalent elements, or those which belong to the same isomorphous groups, under the same head. Thus the formula of garnet is written R 3 " R 2 '"(SiO 4 ) 3 , where R" stands for all those bivalent elements or groups of elements which may replace each other in the garnet molecule ; R" usually is Ca, Mg, Fe, Mn ; and R'" is usually Al, Fe, Cr, Ti, and Mn. TiO 2 may also replace Si0 2 . In the analysis of a garnet the following per- centages were found ; the formula would be calculated as follows : MOLECULAR PER CENT. WEIGHT RATIOS SiO 2 = 41.32 - TiO 2 = 0.16 - - 60 = .689 - 80 = .002 } .691 = 3.02 = 3(R0 2 ) A1 2 O 3 = 21.21 - - 102 = .208 Cr 2 3 = .91 - - 152 = .006 .240 = 1.04 = R 2 3 Fe 2 O 3 = 4.21 - - 160 = .026 FeO = 7.92 - - 72 = .110 MnO = .34 - MgO '= 19.32 - - 71 = .004 - 40 = .483 , .685 = 3. = 3(RO) CaO = 4.94 - 56 = .088 The general formula will then be: 3(R"0)R 2 '"0 3 (R""0 2 )3 or R 3 "R 2 "(R""O 2 ) 3 , or by substituting the elements actually present for R, the for- mula for this garnet is (Mg . Ca . Fe . Mn) 3 (Al . Cr . Fe) 2 ((Si . Ti) 4 ) 3 ; 232 MINERALOGY placing all groups of isomorphic elements within brackets, and the most important element in each group first. It is always under- stood that elements thus written in a mineral formula may replace each other, and they are always written in the order of their im- portance in that mineral molecule. If it is wished to still further simplify the above formula, it is at once seen that both titanium and manganese are present in only very small amounts and may be neglected in the formula. Classification of minerals. Any natural classification of min- erals must consider both the chemical and physical properties of the mineral, but as the physical properties depend to a large extent upon the elements present, the chemical relations of the elements in the mineral molecule are given more weight in the scheme of classi- fication than the presence of any one element. Minerals are therefore classified as chemical compounds, following the order of the natural classification of the elements, but the acid radicle will determine the group ; for example all sulphates are placed in the same large group. The subheads are determined by the character of the salt ; whether it is anhydrous, basic, acid, or hydrated, as well as by a consideration of its crystalline type. By this method of classifica- tion those minerals which form natural isomorphous groups are kept together ; for example, barium, strontium, calcium, and lead sul- phates are placed in the same group of anhydrous sulphates, as they should be, since they crystallize in the same type and are isomorphous. If they were classified from their basic or metallic elements, they would be widely separated in four different groups, even though they are all found in nature under the same condi- tions and often several are mixed in the same crystals. The system of classification here adopted is that of Dana, which may not be in all respects the most desirable, yet most of the min- eral collections in the American museums follow this system in their arrangement. Not taking into account the hydrocarbons, there are eight large groups in this scheme, as follows : I. Native elements. Includes the elements which occur in na- ture in an uncombined state, as gold, copper, silver, sulphur, dia- mond, etc. II. Sulphides, including the arsenides, antimonides, and other similar compounds. Mostly salts of hydrogen sulphide, H 2 S, as galena, PbS ; also including the corresponding compounds of tellurium and selenium. III. Sulpho-compounds, as the sulpharsenites, sulphanti- THE RELATION OF MINERALS TO THE ELEMENTS 233 /S-H monites, etc. Derived from such acids as As ^ S H = H 3 AsS 3 , \S-H as proustite, Ag 3 AsS 3 . The group also includes basic, acid, nor- mal, and meta groups. IV. Haloids, or salts derived from the haloid acids, as HC1, HBr, HI, and HF ; halite, NaCl ; iodyrite, Agl ; bromyrite, AgBr ; fluorite, CaF 2 . V. Oxides, or the combinations of the elements with oxygen. a. The oxides of the univalent metals, as cuprite, Cu 2 O. b. Oxides of the bivalent metals, as zincite, ZnO. c. Oxides of the trivalent elements, as corundum, A1 2 03. d. Oxides of the quadrivalent elements, as quartz, Si0 2 . Also included in this grou are the hydroxides, in which the base is combined with hydroxyl, as brucite, Mg(OH) 2 . The aluminites, ferrites, and chromites, salts of such acids as HA10 2 , as spinel, Mg(AlO 2 ) 2 , or of H(FeO 2 ), as magnetite, Fe(FeO 2 ) 2 , and chromite, Fe(CrO 2 ) 2 , are included in this group, though they must be considered as salts and not oxides. VI. Salts of the oxygen acids. a. Carbonates, H 2 CO 3 , as CaCOs, calcite; basic carbonates, as malachite, (CuOH) 2 COs, and hydrous carbonates, as trona, Na 2 CO 3 . 10 H 2 O. 6. Silicates, titanates, etc. Orthosilicates are the normal salts of the tetrabasic acid, H 4 Si0 4 , as fayalite, Fe 2 Si0 4 . Metasilicates. Orthosilicic acid by a loss of some of its water forms the dibasic acid, metasilicic acid. There are many minerals which are salts of this acid, as enstatite, MgSiO 3 , or leucite, KAl(SiO 3 ) 2 . Diorthosilicic acid is derived from two molecules of orthosilicic acid by the loss of one molecule of water. o-H n . 0-H- H20 ^ (HO) 3 =Si-O-Si=(OH) 3 ; H 6 Si 2 7 . It is an hexabasic acid, an example of which is barysilite, Pb 3 Si 2 O 7 . 234 MINERALOGY Dimetasilicic acid is derived from two molecules of an ortho- silicic acid by the elimination of three molecules of water, or from two molecules of a metasilicic acid by the elimination of one molecule of water. ( J_TT TT i~\ l> i -LJ. XI. Vy ' x /-^ x v "- r i *-* i TT r-4* ^^^v H 2 Si 2 orthosilicic acid O = Siv ___________ r ^81=0 : HjSifQi dimetasilicic acid. \O;H H-Oi/ metasilicic acid Example, Petalite, LiAl(Si 2 5 ) 2 . Trisilicic acid is an acid obtained from three molecules of the orthoacid by the elimination of four molecules of water. Orthosilicic acid O = Si-O-H iH+Qv si /0-H | ;H-O/ Nq r Hi o T - H ~ \Si/ 3 t.?.i ; H-O-Si-O-H; H 4 Si 3 8 = trisilicic acid. H~~O / \ O H | 9 H _ -Si-0 Example, Orthoclase, KAlSi 3 O 8 . In some minerals several silicic acids rnay be present in the mole- cule, and a mineral may be a mixture of two or more of these acids and yet on analysis yield the same percentage results of oxygen and silicon. From inspection alone it is impossible to determine whether the analysis represents a metasilicate or a trisilicate and orthosilicate mixed; to this difficulty must also be added the chances of basic, acid, and hydrated salts being present. For the same analysis several structural formulae of the mineral may be written, all of which will represent the results equally well; thus, andalusite, Al 2 SiO 5 , may be written as an orthosilicate, O = Al - SiO 4 = Al, or as a metasilicate (AlO) 2 SiO 3 , both of which will yield the same percentage composition on analysis. The probable structural formula is derived from a study of the decomposition products of any mineral, as well as by a thorough study of its synthesis where that is possible. In the case of anda- THE RELATION OF MINERALS TO THE ELEMENTS 235 lusite, it decomposes, forming muscovite. The formula of mus- covite is that of an orthosilicate, and when written in the form of a substitution product in the normal aluminium orthosilicate is as follows : Muscovite Andalusite XH /Al = Si0 4 Mg = tremolite. Ca 236 MINERALOGY The alteration and relation of many orthosilicates is easily explained, and their formulae may be written as substitutions in the general formula of an aluminium orthosilicate, in which the aluminium is replaced by other bases. /Si0 4 =Al Aluminium orthosilicate Al\- SiO 4 = Al ; Al 4 (Si0 4 ) 3 . \Si0 4 =Al / / Ca; Ca 3 Al 2 (Si0 4 ) 3 . Garnet = Al< -SiO 4 LQ a \Si0 4 =Al H K Mg Biotite = Al s Si0 / Mg . HKMg 2 Al 2 (Si0 4 ) 3 . \SiO 4 =Al /Si0 4 =(AlF 2 ) 3 Topaz = Al<~Si0 4 = Al ; Al, (A1F 2 ) 3 (Si0 4 ) 3 = Al (A1F 2 ) Si0 4 . \SiO 4 E=Al c. Columbates and tantalates. Salts of columbic acid, HCbO 3 , as columbite, (Fe.Mn) (CbO 3 ) 2 , an orthocolumbate of iron and manganese. Tantalates, salts of tantalic acid, HTaO 3 , as tantalite, Fe(Ta0 3 ) 2 . d. Phosphates, arsenates, and vanadates, etc. Salts of the acids, H 3 PO 4 , H 3 As0 4 , H 3 VO 4 ; as xenotime, YPO 3 ; berzelite, (Ca.Mg.Mn) 3 (As0 4 ) 3 ; and pucherite, BiV0 3 . e. Borates and uranates. Salts of boric acid, as sassolite, H 3 B0 3 ; also biborates, as borax, Na 2 B 4 O 7 . /. Sulphates, chromates, and tellurates. Salts of sulphuric acid, as barite, BaSO 4 . Salts of chromic acid, as crocoite, PbCr0 4 . g. Tungstates and molybdates. Salts of tungstic acid, H 2 WO 4 , as wolframite, (Fe.Mn) W0 4 . Salts of molybdic acid, as wulfenite, PbMoO 4 . In most cases the above acid groups include, in addition to the normal salts, hydrated, acid, and basic salts, as well as complex molecules, the exact structure of which has not in all cases been determined. CHAPTER II THE ORIGIN OF MINERALS THE origin of all minerals must have been from solutions and gases, or the result of the interaction of these on minerals previously separated from solution. It is of very little difference whether the solution is one of water or of fused silicates, homogenous and fluid only at high temperatures. In either case the same laws will apply, and the same physical and chemical laws will hold true whether that solution is but a thimbleful of silicate fused in the electric furnace of the laboratory or contained in the enormous crucibles of nature. Practically the same conditions can be du- plicated in the laboratory with the exception of time and pressure. Time is the factor so essential for the formation of large and per- fect crystals. Most minerals, at least the common species, have been synthe- sized or artificially produced in the laboratory. The exception is a class of minerals which it is believed have been formed under pres- sure and in the presence of water, or they belong to that class which are produced by contact metamorphic conditions, in most cases, as the micas, epidote, topaz, vesuvianite, and amphibole. The artificial products lack water in their constitution, and therefore the conditions under which they were formed in nature have not been reproduced or duplicated in case of the artificial product. Indeed the conditions surrounding the crystallization of any mineral in nature must be excessively complex. The conditions under which individual minerals may form as conditioned by tem- perature, pressure, and other components in the system have been studied in but few cases, and in these the result has been the dis- covery of unknown, unsuspected phases, which have been of great assistance in the explanation of the various isomorphous series, containing the same elements but of different symmetry. In order to arrive at some small conception of the importance and influence of one compound or component in solution on the separa- tion or crystallization of another component of the same system, it is probably best to illustrate by a system in which the relations are 237 238 MINERALOGY as simple as possible and with one which is familiar to all, as the solution of salt in water a two-component system easily tried and from the consideration of which the application of the terms and diagrams may be appreciated. If any crystalline substance is heated and the application of heat is constant and continuous, the temperature of the crystalline solid increases constantly ; in time a temperature will be reached at which the solid passes to the liquid state or the solid is said to fuse or melt. The molecular network or point-system of the crystalline state is broken down. If the temperature of the system when the two phases, solid and liquid, are in contact is measured, it will be found that this temperature is a constant one, providing that the com- pound is chemically pure and the pressure is the same. Thus ice when heated to C. melts and forms water ; this temperature of fusion is the melting point. Water is an exception to the general rule, and the melting point of water is lowered with an increase of pressure, the reverse of the conditions in most other substances. The transition of a solid crystalline substance to a liquid is an abrupt one, and there is no constant decrease of viscosity, with the rise of temperature, to be noted in case of the crystalline substance as is the case with the amorphous solid. In the amorphous solid there is no constant sharply defined temperature of fusion, no abrupt change from solid to liquid. The viscosity simply decreases with the rise of temperature. The line of separation between liquids and solids, when considered from this standpoint, should be drawn between the cyrstalline substances and amorphous substances rather than between solid amorphous substances and the ordinary liquids. The difference between amorphous solids, which are excessively viscous liquids, and the ordinary liquid is very little ; and the amor- phous solid is more closely related to liquids than to crystalline solids. Water and ice are two modifications of the same chemical substance, between which there is an abrupt change or transition from one to the other. Each of these modifications is known as a phase. Water can exist in three phases : solid, ice ; liquid, water ; and vapor or gas, steam, depending upon the temperature and pres- sure. Sulphur may exist in two solid crystalline phases, mono- clinic and orthorhombic sulphur; the transition temperature between these two phases at ordinary pressure is 95.6. Many solid compounds as minerals have different solid phases. Leucite THE ORIGIN OF MINERALS 239 is orthorhombic at ordinary temperatures, but at 500 C. it passes over to an isometric phase. In like manner quartz, when heated to a temperature of 575, /3-quartz is formed, and crystals in passing this inversion temperature are shattered ; if the temperature is still raised, at 800 C. another inversion point or temperature is reached, when a third phase, or tridymite, is the stable form. Different phases of the same substance can exist in equilibrium with each other only when the temperature and pressure are fixed. Ice and water are in equilibrium and can exist in contact with each other at C. and under one atmosphere of pressure. When two phases of the same substance are in contact, for each pressure there is a fixed definite temperature of equilibrium, and for every fixed temperature there is also a fixed pressure of equilibrium. If ice is heated and the heat is applied at a constant rate, the tem- perature of the ice will rise regularly ; as for every gram of ice a con- stant amount of heat is required to raise the temperature one de- gree, this quantity of heat is designated the specific heat of the substance. If the temperature is taken every ten seconds and the curve is plotted with temperature and time as ordinates and ab- scissa, the curve will be found to be continuous and regular until C. is reached, when there is an abrupt change in the behavior of the system, as water, or the liquid phase, begins to form. The rise of temperature is stopped and remains the same, just as long as there is any ice or solid phase present. At this point, Fig. 368, the heating curve runs along horizontally and there is no rise in temperature, since the melting ice absorbs heat in passing in to the liquid phase. This absorbed heat is the heat of fusion; and when water is transformed to the solid phase, ice, the same amount of heat is liberated, and the cooling curve will show the same hori- zontal portion at C. ; such an interruption in the regular trend of the cooling or heating curve is a halting point. Such halting points mark the transition temperatures between two phases of a sub- stance and are caused by the absorption or liberation of heat. The heat liberated by a solid crystallizing is often termed the heat of crystallization; this is a constant source of heat, available, in magmas that are crystallizing, to counteract radiation and serves to prolong the liquid state and to modify the rate of cooling. In such fluid and crystallizing magmas each substance which crys- tallizes and separates from the solution,will have a direct influence upon the temperature at which other substances in turn crystallize; and each compound dissolved in the magma will also influence 240 MINERALOGY the temperature at which crystals of every other compound con- tained in the solution will begin to form. If two substances as salt, NaCI, and water are taken, and the salt is dissolved in the water, a two-component system will result. From a saturated solution of salt in water, salt crystals separate without water, and water will separate as ice without salt ; these conditions are the simplest possible. Salt crystals and ice are the two solid phases and dissolve in each other to the liquid phases. If one substance is dissolved in another, as salt in water, the temper- ature of fusion is always lowered or the temperature of solidifica- tion of both components is lowered. The change in temperature at which each will solidify caused by the addition of the other is known as the lowering of the freezing point. If water at 60 C. is surrounded by a temperature of 30 and its cooling curve is plotted, Fig. 368, the curve will be regular until C. is reached, when ice crystals will form and the temperature will halt at this point as long as there is any water to be transformed to ice and absorb heat, when the lowering of the temperature will proceed in a regular manner, as before. If to the water, or liquid phase, salt be added to, say, an amount of 15 per cent., then if the system be cooled as before, when C. is reached, the curve will show no indication of a halting point and no ice crystals' will form until the temperature has fallen far below the freezing point of water, as the freezing point of water has been depressed by the addition of salt, and ice will not form until a temperature is reached corre- sponding to the amount of salt added. In the case of a 15 per cent, solution this temperature is 12.2. As ice crystals separate, the composition of the remaining solution is constantly enriched in salt and impoverished in water by the ice formed, which is unmixed with salt. As the percentage of salt increases, this lowers still further the freezing point of the remaining solution, (indicated by the curve AB), until the temperature is lowered to or reaches 22.4, where there is a halting point, until the liquid phase disappears. At this point both salt and ice are formed and solidify as a whole ; the temperature, owing to the heat of crystal- lization, remains constant until the liquid phases have entirely disappeared, after which the cooling will proceed in a regular man- ner. If the mixture of salt and water which solidifies as a whole when the temperature of 22.4 is reached be analyzed, its com- position will always be found constant, and it is composed of 77 per cent, of water and 23 per cent, of salt, and no matter what THE ORIGIN OF MINERALS the percentage of the original solution might be, upon cooling either salt or water will separate from the solution as the case may be and continue *to do so, as the temperature is lowered, until the constant percentage mixture of 77 parts water and 23 parts salt is reached at 22.4, when the solution will solidify as a whole. Such mixtures, which have a constant composition and solidify as a whole at definite temperatures under like condi- tions, are termed eutectic mixtures. Fig. 368 is a so-called fusion diagram of a two-component system, water and salt. The plan, one in which temperatures are measured vertically and percen- tages horizontally, is divided into four fields of conditions by the two curves AB and BC and the straight line DE. In field I, 242 MINERALOGY above both curves, is the liquid field, in which all possible mix- tures of water and salt may exist as homogeneous solutions and be in equilibrium at the corresponding temperatures. This field is bounded by the two curves AB and BC. The curve BC is the boundary between homogeneous solution and field III, in which ice crystals and salt solution are in equilibrium. If any point in field I is selected, as p corresponding to a temperature of 20 and 10 per cent. NaCl, at 20 this solution is homogeneous, but if cooled, the 10 per cent, solution will meet the curve AB at a point correspond- ing to 6.7, and at that temperature ice crystals will begin to sepa- rate and the conditions of field III will be reached, in which ice crys- tals are in equilibrium with salt solution. If a point on the opposite side of the diagram, as P, be taken, on cooling, the perpendicular from P cuts the curve CB at 5, at which temperature the solution will be saturated in regard to the salt, and salt crystals will form, when the solution or conditions will be those of field IV, in which salt crystals and salt solution are in equilibrium. As the tempera- ture falls, the solution becomes more concentrated in respect to the water content and the temperature of separation follows the curve BC until B is reached, when both salt and ice separate in eutectic proportions of 77 parts water and 23 parts salt, at a temperature of 22.4. Below the line DE or below the point B or a tem- perature of 22.4, liquid solution cannot exist, and field II is the crystalline or eutectic field, separated from the other three fields in which liquid solutions are possible by the straight line DE, known as the eutectic horizontal. It often happens, and it is indeed the rule, that the temperature will fall below that indicated by the curve of separation without crystals being formed, unless care is taken to prevent it ; such a solution is said to be supercooled. If the 10 per cent, salt solution should fall below - 6.4 without the separation of ice, it would be supercooled and in a metastable condition, the solution being supersaturated as regards water; such a condition could not exist if the least particle of ice were pres- ent. On dropping a particle of ice in the supercooled solution there is an immediate separation of ice, a rise of temperature from the heat of crystallization, as well as an increase of the con- centration of salt in solution, and the whole system comes to a state of equilibrium on the curve of separation. A good example to illustrate supersaturation is a solution of sodium sulphate, Na2SO 4 , 10 H 2 0, dissolved in water and heated in contact with the salt, or saturated at a temperature a little below 32 C. ; if it is THE ORIGIN OF MINERALS 243 heated to a temperature above 32 the anhydrous salt Na 2 SO 4 will separate at 33. The solution is carefully poured off the crys- tals in a flask and the mouth of the flask stoppered with cotton to prevent the access of dust or fine particles of sodium sulphate from the air, which would start crystallization and prevent supercooling or supersaturation. Such a solution may be cooled to room temperature and kept for a long time without the formation of crystals. This solution is supersaturated in regard to the salt Na2SO 4 , 10 H 2 ; if the smallest particle of this salt is dropped into it, the solution solidifies almost at once, with a rise in temperature, and the system soon reaches a state of equilibrium, as between the solid Na2SO 4 , 10 H 2 O and solution. The supercooled solution is in the metastable state, and the tend- ency to form crystals spontaneously is very small ; if cooling is "continued, however, a temperature will be reached at which a cloud of small crystals will form spontaneously, or a number of crystal- line nuclei will form without inoculation with a solid particle, and thereafter the system quickly reaches equilibrium. In the case of igneous rocks, before consolidation they may be said to represent solutions of various components, each of which will have an individual and separate influence on the temperature of sep- aration of every other dissolved component in the magma ; and the freezing curve, or the curve of separation of any crystalline form or phase separating, will be the resultant of all these depressions. Where the number of components is quite large, six or eight, as it is in most magmas, the system becomes excessively complicated. Upon the whole, when cooled to the metastable stage of one or more of the components, a temperature will be reached at which crystalline nuclei will form spontaneously, and the minerals will separate from the magma in the order of their freezing points or saturation for that particular system, and each individual crystal will continue to grow as cooling of the magma continues. In the fusion diagram, fields III and IV, other things being equal, would represent a porphyritic development of one or two species; the individual crystals of each would be contained in a ground mass of fine crys- tals, representing the final disappearance of solution, or the eutectic mixture composed of the last to crystallize, Fig. 369. The eutectic mixture in its crystallization always presents a peculiar, intimate intergrowth of the components. This is seen in the micropegmatitic intergrowths of quartz and orthoclase, Fig. 370. Eutectics are often formed between garnet and quartz ; magnetite and horn- 244 MINERALOGY FIG. 369. Porphyritic Feldspar Crystals in a Fine Grained Crystalline Ground Mass. blende; quartz and tourmaline. Ternary and quaternary, eutectics are also possible. Eutectic consolidation will at times represent a second generation of small crystals, surrounding the porphyritically de- veloped crystals of the first generation. Minerals which are crystallized from molten mag- mas are usually an- hydrous or contain small amounts of water, as the micas, and are termed pri- mary minerals, to distinguish them from secondary min- erals, which have resulted from water solutions, the components of which have resulted from the breaking down of other minerals. The order of crystallization of the primary minerals from magmas is not rigidly fixed, but may vary accord- ing to the composition of the magma, as the . curve of separation of any individual species is the result of the lowering of the freez- ing point of that in- dividual by all the other components of the system, and these may have a greater effect on one than on another. The order of crystallization and the relation of the crystals of one species to those of an another species will depend upon and reflect magmatic peculiarities. There are, however, FIG. 370. Section of a Micropegmatite enlarged, showing Eutectic Structure. THE ORIGIN OF MINERALS 245 certain minerals which ordinarily separate during the early stages of crystallization, or the solution becomes saturated in respect to them long before it is saturated in respect to others. Minerals which are early to crystallize are well formed and exhibit crystal- line outlines in the rock section. They may be contained as inclusions in the crystals of those minerals which form later, as the crystals of apatite in magnetite, or magnetite in feldspar. The crystals of minerals among the last to separate are usually irregu- lar and act as a ground ma.ss, filling the spaces between crystals which have separated at an earlier stage. In the common mag- mas it may be stated that separation takes place with a progressive increase of silica. Those minerals high in silica, as quartz, are near the end products of crystallization. Among the first to separate are metals, sulphides, oxides, and included here are apatite and zircon ; second, the ferromagnesian silicates, as pyroxene, amphibole, and olivine ; third, feldspars, beginning with the basic plagioclases and ending with orthoclase ; and lastly quartz. This order of crystallization must be considered only in a general way, as several may overlap in their periods of separation, and this overlapping may be continued to such an extent that the order of adjacent minerals in the list or order of cyrstalliza- tion is reversed. Magmas homogeneous at high temperatures may upon cooling separate into several portions which are immis- cible at lower temperatures, each of which will have an individual character and composition, even before crystallization has begun. In the crystallization of magmas, the viscosity has a great influence upon the crystals formed and upon crystal growth. With the decreasing temperature there is usually an increase of viscosity, which tends to prevent the formation of centers of crystallization, and is therefore a check generally to crystallization ; and the very viscous magma has a predisposition to solidify as a glass. The fluidity of minerals at their fusing point varies greatly, even within isomorphous groups ; as the potash feldspar, orthoclase, is exceed- ingly viscous at its point of fusion, crystals form in a fusion of its components with great difficulty or not at all ; while the calcium feld- spar, anorthite, crystallizes with the greatest ease from the simple fusion. Viscosity decreases with the increase of basicity, as iron, calcium, magnesium, and sodium promote fluidity, potassium and silica promote viscosity, and aluminium has but little influence either way. Those rocks containing pyroxene, hypersthene, ensta- tite, or olivine are more apt to contain well-formed crystals than 246 MINERALOGY those containing felspathoids, silica or alkali feldspars, when under the same conditions. In the synthesis of minerals it has been proven without doubt that certain minerals are easily formed from a simple fusion, com- posed of their chemical constituents in correct proportions; in all cases these fusions are not highly viscous in nature. The plagioclases, magnetite, hematite, rutile, spinels, corundum, some garnets, leucite, olivine, and enstatite have all been formed from simple fusions in an open crucible. They are all rock-forming minerals, and in nature they have in many occurrences been crys- tallized directly from a fused magma. Mineralizers. On the other hand there is a group of minerals which it has been impossible to synthesize by the open fusion method, as there is contained in their molecule small amounts of volatile compounds as fluorine, water as hydroxyl, or chlorine; or again the fusion at the point or temperature of crystallization is so viscous as to prevent the formation of crystals, when the fusion cools as a glass. Many granites, syenites, and gabbros are excep- tionally well crystallized and contain orthoclase, albite, quartz, amphibole, and micas, minerals which from the experience with open crucible fusions of their chemical components cannot be crystallized, and the formation of crystals requires other substances to be present in the fusion, as water, fluorine, boron, chlorine, tungsten, etc., even though these elements are not a part of the mineral molecule formed. Such elements are termed mineralizers from the role they play in the formation of certain minerals. They do, however, enter the molecule of some of these rock-form- ing minerals in very small quantities, as the micas always con- tain small amounts of hydroxyl and fluorine, apatite contains fluorine and chlorine, and tourmaline contains boron as a molecu- lar essential. Mineralizers are fluxes in that they decrease the viscosi.ty of magmas in the same sense that fluorite is used in many smelting operations to attain the same end, that of forming a liquid slag. They are solvents in the same sense that water is a solvent for salt in the two-component system illustrated, and in each case they lower the fusing point or the temperature of separation to such an extent that it is possible for molecules to form at a much lower tempera- ture and at temperatures far below their actual melting points, as many minerals are unstable at their temperature of fusion and break down into components formed of molecules of a less com- THE ORIGIN OF MINERALS 247 plex nature, and it is not possible for them to form at the temper- ature of their freezing point. Such minerals are termed the low temperature minerals. Their molecules are considered to be more complex than those which form from direct fusion and without the aid of mineralizers, or the high temperature minerals. In the low temperature minerals are included the amphiboles, micas, sodalites, nepheline, tourmaline, topaz, beryl, titanite, quartz, albite, and orthoclase, and also many rare minerals of the pegmatites. In the synthesis of this class a mineralizer must usually be present, either to lower the fusing point or to reduce the viscosity of the melt, while some, as topaz, tourmaline, and muscovite, have never been produced artificially. That mineralizers have been present during the stage of crystal- lization of such rocks as granite, syenite, and pegmatites is shown by the simple fact that the quartz of such rocks always contains numerous cavities holding liquid inclusions, and that the micas con- tain hydroxyl and fluorine. The mineralizer is usually a volatile substance or forms volatile compounds with the bases as the fluorides, and therefore when the magma is extruded they escape, and with their escape the tendency to quickly solidify and the for- mation of glass is increased. Basalts are more often crystallized than are the rhyolites and andesites, as in the latter the mineral- izers have escaped, leaving the magma by nature too viscous to crystallize. The coarse crystalline forms, granite and syenite, are plutonic rocks and have crystallized under conditions which pre- clude any escape of the volatile mineralizers, or they have escaped very slowly. In the process of cooling, those compounds which are the more insoluble have separated first, with the result that the remaining liquid portion is a concentrated solution of the more soluble, and the dykes, veins, and marginal masses connected with some granites and known as pegmatites are the result of and represent the ulti- mate concentration of some of the constituents of the original fused magma. Crystals of pegmatites are large and well formed, and such dykes contain minerals in quantity which are rare accessories in the rock mass as a whole. There seems to be no good reason why the condition should not change during crystallization from that at the beginning, a fused magma, at a high temperature, far above the critical temperature of water, 365 C., to that at the end, when the final product of crystallization may be separated from a solution in water, though hot and under pressure. 248 MINERALOGY Pneumatolysis. Most mineralizers form gases and volatile compounds which penetrate the cracks, cavities, and pores of adja- cent rock formations, where they may be condensed, or by decom- position deposit compounds ; or by the reduction of temperature and pressure deposit compounds carried in solution; or by the direct interaction with the rock mass form minerals, all of which are concentrated near the margin of large intruded igneous masses and extending out as impregnations in the sedimentary formations of the immediate vicinity. Numerous ore deposits are of this na- ture, especially those of tin or cassiterite, where the tin has been concentrated by a squeezing out of the volatile tin fluoride to be decomposed by contact with steam ; it deposits SnO 2 in the open veins and cracks of the formation; this decomposition produces hydrofluoric acid to further react with the walls of the formation to produce such minerals as fluorite and topaz. Minerals formed by the action of gases or volatile compounds are said to be pneu- matolytic or formed by pneumatolysis. The gases most active in pneumatolysis are fluorine, water or steam, hydrogen sulphide, boron, chlorine, and their volatile compounds. Typical minerals formed by this process are tourmaline, topaz, cassiterite, rutile, oxides of iron, micas, fluorite, quartz, sulphides of copper, lead, arsenic, also titanite and axinite, and numerous rare minerals as wolframite, scheelite, uraninite, and allanite. Intimately connected, possibly with the last stages of cooling of large intruded igneous rock masses, is the formation of the so- called contact minerals ; produced by the interaction of and the impregnation of the formation by steam and associated with the heat of contact and pressure. This stage directly follows that of the pneumatolytic action of gases and is usually termed thermal metamorphism, though heat is connected with all metamorphic changes; m this case the temperature is comparatively high ; especially near the contact of the igneous rock intruded. The min- erals formed under such conditions, when the volatile gases are pre- vented from escape and are held under pressure, are of a complex nature when contrasted with those formed where there is a ready escape of gases. The materials at hand for the formation of new minerals are not only those contained in the sedimentary formation under the pro- cess of alteration, but also those elements carried in solution, by the interaction of which at high temperatures and pressure and con- tinued through long periods of time large and most beautifully de- THE ORIGIN OF MINERALS 249 veloped crystalline specimens are formed. In such regions hydrated minerals as the zeolites, kaolinite, etc., are dehydrated, forming feldpars ; silicic acid will replace carbonic acid in limestones ; and large areas of limestone have been almost entirely replaced by such silicates as scapolite, pyroxenes, micas, tourmaline, and feldspars. Other characteristic minerals of contact metamorphism are epidote, garnets, vesuvianite, spinels, wernerite, andalusite, corundum, apatite, biotite, phlogopite, and all those minerals so common in the granular limestones. Practically there is no sharp line to be drawn separating the two processes of thermal or contact metamor- phism and pneumatolysis ; one begins where the other leaves off, and the two are so intimately related in the formation of some contact minerals as to be quite impossible of separation. The volatile matter given off by hot intruded magmas may pos- sibly be exemplified by a study of the gases escaping from active volcanoes, though the difference here would be that of a great de- crease of pressure. With the relief of all pressure, only those com- pounds would exist which are stable at atmospheric pressures, compounds possibly quite different from those which are in solu- tion and which are stable under high pressures and where the volatile gases do not escape freely. The common gases emitted from volcanoes, solfataras, and fumaroles are hydrogen sulphide, sulphur dioxide, carbon dioxide, hydrochloric acid and volatile chlorides and fluorides, steam, nitrogen, and other gases in much smaller quantities. Many minerals are formed as sublimates by the direct condensation and interaction of these gases. They are usually simple in their molecular structure, in contrast to those formed at depths and under pressure. Sulphur is usually present, a product formed by the interaction of S0 2 and H 2 S, also sulphates and chlorides, as NaCl, PbCl 2 , and where the temperature is high, the chlorides are decomposed, forming oxides as melaconite (CuO), cuprite, magnetite, and hematite; all these minerals are known in the lavas around Vesuvius. Hot Solutions. After the cooling of an injected magma has progressed to such a degree that it is possible for the water to exist as such or the temperature has fallen below 365, the critical tem- perature of water, then many minerals are formed or deposited from the water solutions. Water under pressure dissolves many compounds with ease which at the surface or under normal temper- ature and pressure are but slightly soluble or are considered to be insoluble. The solubility of many substances is greatly 250 .MINERALOGY increased by other components in the solution, especially CO 2 , H 2 S, HC1, HF, or alkalies. All substances, even the most insoluble, are dissolved in slight amounts; insolubility is only a relative term. Gold itself is soluble in water and especially so in the presence of ferric chloride, and solutions of choloidal gold will retain the metal without sepa- ration for a long time. Solubility is also increased with an increase of pressure, provided that the total volume of solute and solvent is decreased by solution. Hot flowing waters under pressure are therefore very complex solutions, and are usually nearly saturated with compounds which at the surface would be considered insoluble. When such solutions, flowing through the veins and fissures of the adjacent formation, gain access to regions of lower temperatures and pressures, they are in a state of unstable equilibrium, and some of their components are deposited. As the directional flow is constant for long times, these streams of solutions serve to concen- trate the soluble components in the fissures and veins, where they are deposited, often in definite order and at times filling completely the original veins and fissures along which the solutions flowed. Many ore deposits filling veins, fissures, and pipes have been con- centrated by this method ; and often where such heated solutions reach the surface, as at the Steamboat Springs of Nevada, the depo- sition of sulphides has been noted. Pyrite, chalcopyrite, galena, arsenides, antimonides, and many minerals of like character have been deposited on the walls of channels by the hot solutions flow- ing through them. When minerals are crystallized under high pressures, the molecules are compact, and the minerals formed are of a phase in which the specific gravity is high ; as quartz, with a specific gravity of 2.65, will form' rather than tridymite, with a specific gravity of 2.3. Tridy- mite occurs in surface lavas where it has crystallized at reduced or atmospheric pressures. High pressures, other things being equal, will induce the formation of pyrite, with a specific gravity of 5 rather than marcasite with a specific gravity of 4.9. As solids in the crystalline phase occupy less space or have a higher specific gravity than the amorphous phase, high pressures will tend to dis- solve the amorphous phase and to redeposit the chemical com- pound as crystalline. Large areas of amorphous limestones have been thus crystallized under pressure. Ordinary obsidian when crystallized will occupy less space and during crystallization will shrink from 3 to 11 per cent, of its original volume. Garnets, feld- THE ORIGIN OF MINERALS 251 spars, micas, pyroxene, amphibole, epidote, spinels, and andalusite are minerals which form under high pressures ; the latter are dense minerals and differ entirely from minerals formed at or near the surface, where hydrated minerals are the rule, a class quite unstable under high pressures and heat. Under such conditions the latter are quickly transformed to the more stable compact molecule. If the minerals which have been formed at depths are brought to the sur- face, they become the less stable phase, when they are subject to change and decomposition, either through the solution of some of their constituents, or oxidation, or by replacement of some ele- ments by others. The action of ground waters. Water falling as rain passes through and over the soil, following the course of least resistance, constantly taking up and carrying along soluble compounds. As CO 2 is absorbed from the soil and the air, its solvent power on other carbonates, as calcium carbonate, is thereby increased ; such car- bonates in solution are considered as the bicarbonates and are more soluble than the normal salts. There is an area near the surface, usually termed the area of oxi- dation or weathering, in which the pores and cavities of the soil and rocks are not completely filled by the percolating waters. In this area oxygen is an active agent, and many minerals are oxidized as they are carried in solution, or oxidation is the cause of precipita- tion. In this area sulphides, as pyrite, are oxidized to sulphates, forming ferric and ferrous sulphates, both of which are soluble and carried in solution by the descending ground waters, to act as power- ful reagents in the transformation and solution of other minerals. In the area of oxidation, such processes as carbonation and hydra- tion, in addition to oxidation and solution, are active, resulting in the formation of carbonates and many hydrated minerals and hydroxides, as limonite and the oxides of manganese, and alumin- ium, as well as sulphates, arsenates, and phosphates. In this region carbon dioxide can replace silica, the silica being carried in solu- tion to be again deposited either as the amorphous form, opal, or as quartz, according to conditions. This reaction is the reverse of that which takes place at greater depths and under the influence of heat and pressure, where silica replaces CO 2 . Upon the whole minerals in the zone of oxidation or weathering are decomposed and disintegrated, forming products which with the addition of water and oxygen have increased in volume, and in the production of which hydration and oxidation are most important processes. 252 MINERALOGY In the percolating ground waters many sulphides are soluble, and especially so as these waters are acid in some cases and alkaline in others. These solutions finally, in their descent, reach the area where the pores, fissures and cavities of the rocks are completely filled; and the solution joins that reservoir of ever flowing water termed the ground water; where through the action of diffusion and flowage, solutions of various substances are mixed as reagents in a beaker, with the resulting precipitations. Here, however, the walls of the containing cavities are active agents and by the inter- change of their elements enter the reactions as important factors in the process of chemical replacement, or metasomasis. The products of the oxidation of pyrite are varied, according to the conditions and the amount of oxygen available. FeS 2 + 6 O = FeS0 4 + SO 2 , which yields a solution of ferrous sulphate and an acid solution. 2 FeS 2 + 14 O = Fe 2 (SO 4 ) 3 + SO 2 , yielding a solution of ferric sulphate and an acid solution. This solution may interact with more pyrite, FeS 2 + Fe 2 (SO 4 ) 3 = 3 Fe(S0 4 ) + 2 S, yielding ferrous sulphate and free sulphur, also hydrogen sulphide and sulphites may be produced. Ferrous sul- phate in solution is a powerful reducing agent and results in many cases in the precipitation of metals, as silver and copper, and as oxides, as cuprite. Many deposits of iron ore have resulted from the interaction of solutions of iron with carbonates, in which the iron has replaced the calcium of limestones and shells. CaCO 3 + FeS0 4 = FeCOs + CaS0 4 , when siderite and gypsum are formed. It is by means of a similar replacement or interaction of solutions containing sulphates resulting from the oxidation of sulphides near the surface that such carbonates as smithsonite, rhodochrosite, witherite, cerussite, azurite, and malachite are formed ; these min- erals may also be formed from solutions of carbonates. The more soluble sulphate gypsum may be replaced by barium, strontium, or lead sulphates. The minerals barite, celestite, and anglesite are often precipitated on the walls of fissures and veins as the result of the intermingling of solutions. Minerals formed by chemical replacement occur usually as lens-shaped masses embedded in the rock which has been the cause of their formation. Large areas of limestone have been changed to dolomite by the replacement of calcium by magnesium. Most of the galena and sphalerite deposits of the Mississippi Valley have resulted from replace- ments in limestones. THE ORIGIN OF MINERALS 253 The surface sulphides of many deposits have been completely oxidized. Most of the products of oxidation are carried off in solu- tion ; only a small proportion remaining to indicate the nature of the original minerals. The red oxide of iron, hematite, is especially characteristic of such conditions and often covers the lower un- oxidized sulphides like a cap ; from its position it has been termed the " iron cap." The upper oxidized surface areas of ore deposits are also termed the gossan. Minerals characteristic of the gossan are the carbonates, hydrates, sulphates, arsenates, and oxides. The depth of these oxidized minerals, or the gossan, will depend upon climatic conditions, the nature of the formation, and the depth of the ground water. The oxidized areas gradually give way to the region, in ore deposits, termed the areas of secondary enrich- ment, where the sulphides have not only not been oxidized, but have been increased in some of their constituents. The metallic sulphides carried down in solution from the gossan above are pre- cipitated and added to the original metallic content of sulphides be- neath. That the waters descending from the oxidized areas above do carry sulphates in solution is abundantly proven by large num- bers of analyses of mine waters, and also by the fact that in many mining regions the mine waters carry copper sulphate in such quan- tities that it pays to precipitate it from the solutions with scrap iron. Of the metallic sulphides, pyrite, marcasite, and pyrrhotite are the most readily oxidized, and when in contact with solutions containing copper sulphate they precipitate the copper as sulphide, forming chalcopyrite. At the Copper Queen Mine, at Bisbee, Arizona, pyrite too poor in copper to pay for smelting was used to fill old stopes, is now, after some ten or twelve years, remined and smelted, having collected nearly 10 per cent, of copper by precipita- tion from the mine waters through replacement of the iron. Pyrite not only serves as a precipitant for copper^ but also for lead, silver, zinc, and other elements less basic. By this process of oxidation and precipitation the area of second- ary enrichment, or the unoxidized sulphides directly underneath the gossan, contains not only the valuable metals which were their original content, but added to this by replacement is a large propor- tion of that formerly contained in the gossan above. The ore in many mines has been found too poor to work when the regions be- low the areas of secondary enrichment have been reached. This process is therefore one of prime importance, and must be consid- ered in the valuation of ore deposits. Surface waters and spring 254 MINERALOGY waters are complex solutions of carbonates, sulphates, and chlorides of the more soluble bases, as the alkalies, alkali earths, iron, man- ganese, aluminium, as well as silica and a large number of other ele- ments in very small quantities. The amount of solid residue in the ordinary natural waters will vary greatly, depending upon the nature of the soil and the geological formations over and through which the waters flow. Rivers of limestone regions are usually high in their content of calcium and magnesium carbonates, as these carbonates are carried into solution as bicarbonates and they are termed hard waters. Such waters are not saturated solutions, containing in solids from 10 to 40 parts to the 100,000, unless greatly concentrated by evaporation or by the loss of carbon diox- ide, when the normal carbonates may crystallize or be deposited, as is the case in the formation of stalactites and stalagmites in caves and the calcite cement of some conglomerates. Though the amount of dissolved solids in a river water seems very small, yet when the contant flow is considered, enormous quantities of soluble com- pounds are carried in solution. It has been estimated that the St. Lawrence at Ogdensburg, New York, having a flowage of 248,518 cubic feet a second, and a salinity of 13.2 parts per 100,000, will transport by this point, annually, 29,278,000 metric tons of salts in solution. Taking into consideration the area drained, exclusive of water areas, this is equal to, if evenly distributed, 102 tons of matter carried off in solution per square mile each year. The vol- ume of the ocean is so great that its saline content is not apparently increased by these enormous quantities of dissolved salts which are being poured into it by every river; but if at any time such waters are confined and evaporation equals or surpasses the annual addition by rivers and rain, the inclosed lake, basin, or arm of the sea will become concentrated, as in the case of salt and alkali lakes of arid regions. When cencentration advances to saturation, their salts are usually deposited in a definite order, forming stratified deposits, following the order of saturation in regard to the various minerals separated. There is therefore in such saline deposits an order or sequence from the bottom to the top, except where this order may have been modified by some peculiar local characteristic or condition, caused by an unusual component in the solutions. When ordinary sea water is concentrated to about one tenth of its original volume, crystals begin to form. In the normal concen- tration of sea water these crystals are gypsum, and gypsum usually forms the lower stratum of such deposits. Anhydrite may re- THE ORIGIN OF MINERALS 255 place the gypsum deposit, as gypsum is transformed to the anhy- drous sulphate as concentration advances, being induced by a raise of temperature and a concentration of sodium chloride. On top of the calcium sulphate, halite or sodium chloride is deposited, and when finally the mother brines reach the point of saturation for the more soluble sulphate and chloride of magnesium, then double salts are precipitated, as at Stassfurt in Prussia, where some thirty species of minerals have separated from a concentrating brine. Rock salt and gypsum or anhydrite are constant companions, their positions indicating their separation from concentrating brines, though deposits of gypsum will occur without the salt, or it is often interbedded with clay and salt, indicating periods of changed con- ditions in the concentration of the mother brine caused by an influx of the sea water or by periodic additions of a dilute solution. Often the concentration has never reached the stage when rock salt is deposited, and in such cases the deposit of calcium sulphate will exist unassociated with the usual stratum of halite ; or again the salt may have been entirely carried away in solution by the ground water, as salt springs are not uncommon. The minerals of saline deposits include borates, carbonates, chlorides, nitrates, and double salts with various amounts of water of crystallization. They are all quite soluble in water, and as concentration of natural solutions is favored only in very dry climates, they are all characteristic of arid regions. CHAPTER III PHYSICAL PROPERTIES Cleavage and fracture. When a mineral is broken by the blow of a hammer on a sharp-edged instrument, as a cold chisel, held on the specimen, it either breaks in a smooth plane face or irregu- larly. The former is cleavage ; the latter is fracture. Cleavage is caused by the separation along and between layers of molecules. Taken in this sense, only crystalline minerals may possess cleavage. FIG. 371. A Crystal of Galena, showing the Perfect Cubical Cleavage. Missouri. Aurora, Cleavage is named from the crystal form to which the separation is parallel, and is represented by the letter representing the form, as cleavage m = prismatic or parallel to the unit prism, c = basal, etc. Galena and halite have cubical cleavage, Fig. 371 ; cal- cite, rhombohedral ; spodumene, prismatic, etc. A perfect cleav- age is one in which the plane face obtained is smooth, even, and polished and is generally easy to obtain but not necessarily so; 256 PHYSICAL PROPERTIES 257 such a cleavage is the cubic cleavage of galena, in which it is almost impossible, even on grinding, to obtain particles not cubical in form. Cleavage faces may be highly polished and smooth, even though difficult to obtain, as the rhombohedral cleavage of quartz. Cleavage surfaces parallel to the faces of the same crystal form are all of the same character ; they possess the same luster and are obtained with the same ease, as the three directions of the rhombo- hedral cleavage in calcite, or the four directions of the octahedral cleavage of fluorite. In anhydrite, where there are also three cleav- age directions at right angles, they are obtained with unequal ease and each differs from the other in luster. In gypsum there is one very perfect cleavage, parallel to the clinopinacoid, cleavage b, and another parallel to the orthopinacoid, cleavage a, which is obtained with more difficulty; the resulting flat cleavage plates are parallel to the easy clinopinacoidal cleavage, and are bounded by two straight edges parallel to the less easy orthopinacoidal cleav- age. This is also the usual shape of cleavage fragments in ortho- clase. The comparative ease of coexisting cleavages materially influence the general shape of the fragments into which a mineral breaks. The descriptive terms as applied to cleavage are: per- fect, as the rhombohedral cleavage of calcite ; distinct, as the pris- matic cleavage of rutile ; imperfect, as the basal cleavage of beryl or apatite ; traces or indistinct, as the cubic and octahedral cleavage of pyrite ; difficult, as the cleavage of quartz or tourmaline. When the cleavage pieces are flat and it is possible to cleave very thin laminae, the cleavage is said to be micaceous, as in muscovite or biotite. The laminae are described as flexible when they can be bent, even though they may crack, but without parting, as in chlorite; elastic when they bend with an even curve without cracking and on being released spring back to their original flat condition, as in muscovite. The laminae are brittle when they break easily on bending, as in margarite; tough in one direction and brittle in another, as in gypsum. Parting is not a true cleavage, but is the result of pressure or strain to which minerals have been subjected and is therefore not characteristic of all specimens of the same species, but is peculiar to localities and formations and not necessarily confined to crys- tals. Some garnets have a parting parallel to the rhombic dodec- ahedral faces, others have no indication of it. Magnetite and franklinite have an octahedral parting. 258 MINERALOGY Fracture is where the specimen breaks, not along a smooth plane face, but irregularly in no definite direction. The appearance of the uneven surface of fracture is also characteristic, and the follow- ing terms are used in describing the fracture of minerals: Conchoi- dal, when the break results in curved or warped surfaces, as in glass, chrysocolla, or flint, Fig. 372. Subconchoidal, when the curves are not well marked, are uneven, and the surfaces are slightly rough, as in most minerals. Hackly, when the roughness consists of sharp points, as in copper and most metals. Splintery, when the fracture FIG. 372. Obsidian, showing a Conchoidal Fracture. shows a fibrous structure, as in some steatites. Scaly, where the mineral is formed of fine crystal scales, as in lepidolite. Tenacity and hardness both depend upon cohesion. The force necessary to overcome this attraction of one molecule for its neighbor will vary with the molecule and the direction in the crys- tal. A sectile mineral may be cut with a knife, and the shavings remain whole and possibly curl like the shaving of a quill or horn, as graphite, molybdenite, and most micas and metals. A mineral is malleable when it flattens on hammering and spreads out, increasing in area without cracking, as lead, silver, or tin; ductile when it may be drawn out in wire, as copper, silver, iron ; brittle when on hammering it breaks down in a powder as do most minerals, though the ease with which this occurs is modified by such terms as tough, as rhodonite; soft, as wad; friable, as kaolinite. PHYSICAL PROPERTIES 259 Hardness is an uncertain term, but in mineralogy it may be taken to indicate the ease or difficulty with which a mineral may be scratched. It is a directional quality and not only varies with the crystal form, but with the direction on the same crystal face. The hardness of cyanite when tested on the macropinacoid parallel to the vertical axis is 4-5, but tested on the same face at right angles to this direction it is nearly 7. When the hardness is tested in all directions on a face like that of cyanite, and the values plotted in a curve, this curve will be found to conform to and reflect the symmetry of the face, Fig. 373. Where it is wished to determine the hardness with some degree of ac- curacy, the sclerometer is used, the principle of which depends upon weighting a sharp point of a very hard substance, as a pointed diamond, until it just scratches the surface of the specimen when drawn across in any particular direc- tion in which the hardness is wished to be determined. The weight of the load which just produces a scratch is taken as a measure of the hardness. Again, the material removed by the point under a constant load after being drawn across the face a determined num- ber of times may also be taken as a measure of the hardness, but here the hardness will be inversely as the weight of material re- moved. The loss of weight or the material removed is determined by weighing the crystal. Hardness as a test in the identification of mineral species is deter- mined by reference to a series of minerals arranged in a table, in ascending scale, from 1, the softest, to 10, the hardest. The series of ten test minerals is known as Mori's scale of hardness, a purely arbitrary scale and in no way representing the true relation, as the difference between 9 and 10 in the scale is ever so much greater than the difference between 1 and 2. Moh's scale of hardness below is also expressed in terms of hardness as determined by the sclerometer, in which the hardness of the sapphire is taken as 1000. FIG. 373. Curve of Hardness on the Cube Face of Fluorite. 260 MINERALOGY I Talc 1.13 6. Orthoclase ... 191 2. Gypsum 12.03 7. Quartz .... 254 3. Calcite 15.3 8. Topaz .... 459 4. Fluorite 37.3 9. Sapphire . . . 1,000 5. Apatite 53.5 10. Diamond . . . 140,000 The specimens used for the scale should be crystalline, cleavable, and as nearly transparent as possible. In testing a mineral for hardness, a sharp point of the mineral in the scale or test mineral is pressed firmly on a smooth surface of the mineral to be tested and drawn across with a quick move- ment. The harder the mineral the more pressure will be re- quired to make the scratch. Care must also be taken not to mis- take the mark left by a soft mineral on a harder surface as a scratch. This mark, like a chalk mark, may be easily rubbed off, while a scratch may be tested by drawing the finger nail across it. Minerals of the same hardness will scratch each other when tested in this way. In ordinary cases, as the determination of hardness for use with the blowpipe tables, hardness above or below 3, 5, and 7 is an im- portant point. If a mineral will scratch a copper coin, it may be considered as above 3 in hardness, and failing to scratch glass it would be below 5 ; failing to scratch quartz and scratching glass, it would be between 5 and 7 in hardness. With experience the approximate hardness of a specimen may be determined by the ease or difficulty with which it is cut or scratched by an old knife- blade or file. When the knife fails to have any effect on the speci- men, it is above 6 in hardness. In testing the hardness of a min- eral, care must always be taken that it is as pure as possible and free from decomposition products, remembering that impurities, as sand, etc., will cause a soft mineral to appear much harder than in reality it is, and by decomposition minerals which when unaltered are very hard, as corundum or andalusite, will appear softer. Specific gravity = G, is the expression in figures of the ratio of the weight of unit volume of the substance to the weight of unit volume of water at 4 C. A 'mineral in which G = 2.65 (quartz) will weigh 2.65 gm. per cubic centimeter, since one cubic centimeter of water weighs one gram. Knowing the specific gravity of any mineral or rock, it is an easy matter to determine the weight of any cubic amount, as a yard or a foot. For the identification of min- erals, the specific gravity is determined approximately to the second PHYSICAL PROPERTIES 261 decimal place. Various specimens of the same species will differ according to their purities, but chemically pure specimens of a sub- stance determined under the same conditions will not vary in their specific gravity. The specific gravity of most minerals, including most of the sili- cates, will lie between 2.25 and 3.5. Minerals with metallic luster are usually high and will lie between 4.5 to 10, while the specific gravity of the naturally occur- ring metals reaches as high as 23 in iridium. The specific gravity of ice is 0.92. Two general methods are followed in the de- termination of the specific gravity : (a) weighing the substance in air and weighing it in water ; (b) suspending the substance in a liquid of the same specific gravity, then determining the spe- cific gravity of the liquid by weighing. (a) There are several modifications of this first method, depending upon the accuracy required, or the solubility and physical condition of the material. I. Joly balance. A quick but only an ap- proximate method. The balance, Fig. 374, con- sists of a spiral spring S, two scale pans P and P', a long mirror graduated in units and tenths, a white bead b, just above the top scale pan, to assist in reading the scale, and a movable bracket, which supports a beaker of water. If it is wished to determine the specific gravity of a crystal of quartz, as an example, three readings are neces- sary, and in each reading the lower glass scale pan P' should be submerged to the same depth in the water, should not touch the walls of the beaker, and should be free of air bubbles. The first reading is taken with both pans empty and the lower one submerged in the beaker of water ; the eye is held in such a position that the bead b will exactly cover its image in the mirror = C = 2.8 in this case. The specimen is now placed in the top pan P, care being taken that with the extra weight it does not sink in the water and get wet, increasing its weight ; the beaker is now lowered until the pan P' rests at the same depth in the water as before, when the second reading is taken = 16.1. The specimen thoroughly wet is now placed in the FIG. 374. Joly Bal- ance. 262 MINERALOGY lower pan P'. There should be no air bubbles sticking to it ; the beaker is now raised until equilibrium is established, with the lower pan at the same depth in water as before, when the third reading is taken = 11.1. To determine the specific gravity: G = the weight in air = W = 16.1 - 2.8 13.3 loss of weight in water W w 16.1 11.1 5 = 2.66. The specific gravity of pure quartz is 2.653. In this method the specimen used should weigh between two and three grams. The chemical balance may be substituted for the Joly balance and accurate weighings made, with a resulting increase in accuracy. II. Pycnometer method. The pycnometer, Fig. 375, is a small flask of usually 10 cc. capacity, having a nicely fitted ground glass stopper with a capillary bore running through it. The weight of distilled water which exactly fills the pycnometer at 20 C. is determined once for all = C. In each case the pycnometer is weighed empty and dry = P. The specimen is powdered to avoid any internal cavities and about two and one half grams are placed in the pycnometer, it having been dried at 100 C. when there is no danger of loss at that temperature, and carefully weighed = S. The pyc- nometer is now filled about half full with distilled water and very carefully boiled for a few minutes to expel any air that may be included in the powder. After cooling, the flask is filled with recently boiled distilled water, care being taken that none of the sample is lost and that the pycnometer is filled to the top of the small capillary bore when the stopper is in place, and cooled to 20 C., when the whole is. weighed = D. S = sample and pycnometer weighed together. S P = W = weight of sample in air. (S + C) D = loss of weight in water. FIG. 375. Pycnometer. S-P (S + C) - D' PHYSICAL PROPERTIES 263 When all precautions are taken, this method will yield results accurate to 1, in the fourth decimal place. When the substance is soluble in water or affected by water, some other liquid than water is used the specific gravity of which has been determined, as benzene or carbon tetrachloride. The experiment is carried out as with water, but the result must be multiplied by the specific gravity of the fluid used in order to re- duce it to terms of water as the unit. III. The suspension method. The Westphal balance and the use of heavy liquids. The advantage of this method is that the gravity of small crystals or fragments may be determined with accuracy to the third decimal place as in the pycnometer method. The method consists in placing the fragment in a liquid, the spe- cific gravity of which is higher than that of the fragment, which will then float on the surface of the heavy liquid. The fluid is now diluted carefully with a lighter one, stirring after each addition; a point will be reached when the fragment of mineral will neither float nor sink ; the specific gravity of the liquid and fragment are under these conditions identical and the fragment remains suspended. The specific gravity of the fluid is now deter- mined with the Westphal balance or the pycnometer when accu- racy is required. The heavy solutions in general use are : Thoulet's solution. A solution of potassium iodide, KI, 1 part, and mercuric iodide, HgI 2 , 1.24 parts, dissolved in an excess of distilled water. The solution is then evaporated on the water bath until a fragment of fluorite floats on it ; on cooling, the gravity rises to the maximum 3.196. It is then filtered, and well corked, as on exposure to the air it absorbs water and its specific gravity will fall to 3.1, where it remains constant. It is a convenient solu- tion, as it may be diluted with water to any extent, and if left stand- ing exposed, the water will evaporate until the specific gravity reaches 3.1 ; by evaporation on the bath it may again be brought back to 3.196. If darkened by the separation of iodine, this may be corrected by adding a little mercury during the evaporation. Methylene iodide, CH 2 I 2 , G = 3.315 at 20 C., is a little heavier than Thoulet's solution, but has the disadvantage of not being miscible with water, and benzole must be used to dilute it. It does not attack metals as does the mercury solution. The heaviest solution of all is that suggested by Penfield, silver thallium nitrate, which is liquid at 75 and has a specific gravity of 4.5 and may be diluted with water. 264 MINERALOGY Where only an approximate determination is all that is required, as in the identification of minerals, the fragment is placed on the liquid, and the liquid is diluted until suspension is reached, then small fragments of minerals, the specific gravity of which are known, are used as tests. Thus if calcite sinks and quartz floats, the spe- cific gravity of the mineral to be determined must lie between 2.65 and 2.72 or not far from 2.70. A list of convenient test minerals is as follows : Heulandite 2.20 Calcite 2.72 Analcite 2.26 Prehnite ...... 2.87 Gypsum 2.32 Datolite 2.95 Leucite 2.47 Tremolite 3.00 Orthoclase 2.56 Actinolite 3.10 Quartz. . . . . . .2.65 Fluorite 3.18 When accuracy is required the Westphal balance, Fig. 376, is used. This consists of a bob n which just balances the arm H in air. The arm is divided into a decimal scale. Three sizes of weights are furnished with the instrument, A = units, B = .1 and C = .01, when hung on the hookas A 2 , and corresponding tenths of these values when on the arm. In the determination, the bob is immersed in the liquid and the balance arm weighted to balance, when the specific gravity is read directly from the weights and the arm. The separation of minerals with heavy solutions. The different mineral components of a rock such as granite may be separated by the use of heavy liquids. The specimen is ground only sufficiently fine to insure each particle as consisting of one mineral species only; the fineness necessary is determined by an examination of the powder with a microscope. The sample is then charged in chamber A of the separatory apparatus, Fig. 377, and Thoulet's solution, density 3.00, poured in, stoppered and shaken; all acces- sory minerals, as zircon, apatite, magnetite, amphibole, and tour- maline, will settle in the chamber A, while the others will float on the surface of the liquid. The heavy minerals may, after settling, be drawn into B by means of the cock C, and finally after shaking a second time, out at the bottom by turning the cock C'. The solu- tion in chamber A is now diluted until calcite just sinks, when it is again shaken, allowed to settle, and all minerals above 2.72, as the micas, are drawn off. The solution in A is again diluted until quartz just sinks, which is separated from the orthoclase. PHYSICAL PROPERTIES 265 Where samples for analysis are required it is advisable to pass the material through the separator several times, care being taken FIG. 376. The Wesphal Balance. to have the specific gravity of the heavy liquid nicely adjusted to the specific gravities of the minerals to be separated. Terms used in the description of mineral aggregates. Indi- vidual crystals, even though of the same species and combinations of the same forms, may differ greatly in appearance. The general shape of a crystal will depend upon the crystal form which predominates in the combination; thus various habits arise which are described by the following terms : Tabular is when a pair of parallel faces, as the base or pinacoid, predominate. The crystal is short in one direction and extended in the other two, as in some barites, tabular parallel to the base, Fig. 378. Prismatic is when the individuals are elongated in the direction of any one axis, usually the vertical axis, as in quartz. Short and stout prismatic crystals are said to be of columnar habit, as beryl. FIG. 377. 266 MINERALOGY Acicular is where the prismatic habit is so accentuated that the crystals are long and needle-like, as stibnite or rutile. FIG. 378. Tabular Barite and Acicular Stibnite. Felsobanya, Hungary. When still more attenuated or hairlike, they are said to be capillary, as millerite, Fig. 379. Fibrous is when the fine hairlike crystals are parallel in arrange- ment, and usually easily separable, as asbestos, Fig. 380. All the above habits may be illustrated by specimens of the same mineral species. The surfaces of crystal faces and mineral aggregates differ in appearance and the following de- scriptive terms are in use : Striated. Crystal faces are often crossed by striations, which are always parallel to some definite crystallographic direction and FIG. 379. Millerite, Antwerp, New York. Jefferson County, PHYSICAL PROPERTIES 267 often serve for the identification of zones or individual forms. They are of two varieties : those caused by alterations of growth, between two crystal forms and therefore parallel to their inter- section or edge, as the horizontal striations on the prism faces of quartz. When the striations are very marked, the crystal is said to be furrowed. Twinning striations differ from oscillatory striations, or those caused by the alterations in growth between two forms, in that they represent the composition plane between twins which FIG. 380. Asbestos. Thatf ord, Quebec, Canada. penetrate and pass through the body of the crystal. They will therefore appear on cleavage pieces as well as on crystal faces, either singly, as in the Alaska epidotes, or often repeated as parallel lines, as in the plagioclases. Vicinal faces. It has often been observed that each face of a simple form, which may be represented by parameters or in- dices of normal value, as the cube or octahedron in the isometric system, is replaced by a low and very flat pyramid. The number of faces of the pyramid will depend upon the symmetry of the face replaced in the cube four and in the octahedron three. These flat vicinal faces intersect the axes at long distances and their indices, contrary to the general rule of simple indices, are large and indefinite, though the faces conform to the symmetry of the type 268 MINERALOGY and represent always the more general form of the type. Vicinal faces are common on fluorite, where the face of the cube seems to be replaced by a very flat tetrahexahedron, Fig. 381. When a zone of vicinal faces occurs, the crystal appears rounded or bounded by curved faces. Drusy. A surface formed of numerous small individual crystals and therefore dull is said to be drusy. Drusy surfaces may be caused by a sec- ondary mineral produced by surface decomposition, or it may be the result of very fine crystals usually in parallel position, repre- senting a second generation of the same species; or it may be caused by a parallel or radiated aggregate of fine crystals, each terminated at the surface, as in crusts of millerite or smithsonite. Internal structures. Granular is where the individual crys- FIG. 381. A Cube of Fluorite, showing Four Vicinal Faces replacing the Cube Face. FIG. 382. Calamine with Radiated Structure. Franklin, New Jersey. tals are equidimensional and irregularly packed together, the indi- viduals being distinguishable by the unaided eye. Massive or compact. A granular structure in which the indi- viduals are not to be distinguished by the unaided eye. PHYSICAL PROPERTIES 269 Powdery. Is when the fine individuals are easily parted and break down under slight pressure. Lamellar. Is when the individuals are flattened and in paral- lel position and easily parting, as some talc. Foliated is an equiva- lent term, only the laminae are thicker. Radiated. Formed of individual crystals, usually acicular, which radiate from a point, the nucleus of crystallization, like the sticks of a fan. Sometimes termed fan-shaped, as calamine or wavellite, Fig. 382. Reticulate. Individual prismatic crystals arranged like lat- ticework, with definite twinning angles, as the rutile from Tavetsch, FIG. 383. Reticulate Rutile, enlarged. Tavetsch, Switzerland. Switzerland, Fig. 383, or irregularly matted, as the cerussite from Cornwall, England. Dendritic. Branching forms deposited from solutions, in cleavage cracks and rock fissures, as pyrolusite or metallic copper, Fig. 384. Wiry. Like wire, usually native metals, as silver or gold, also in sheets or leafy. Nuggets. Are irregular rounded or rolled lumps, usually applied to the precious metals, as gold, silver, or platinum, Fig. 385. Nodular. Applies to rounded individual masses of minerals, as the rounded balls of pyrite occurring in some clays, Fig. 386. 270 MINERALOGY FIG. 384. Dendritic Psilomelane. Leadville, Colorado. Geodes. Are cavities in clays or other formations, which have been encrusted with a wall of quartz or other mineral, and which FIG. 385. Platinum Nugget. Nischi- Tagilsk, Siberia. FIG. 386. Nodular Pyrite. separate as a hollow mass, the interior walls of which are usually studded with crystals, Fig. 387. Small almond-shaped steam cavi- ties occurring in lavas are often filled by minerals, deposited from PHYSICAL PROPERTIES 271 solution ; such bodies of minerals are termed amygdules and the structure amygdaloidal. FIG. 387. Geode studded with Quartz Crystals. Pisolitic. Spherical forms with concentric shell-like structure, as in some calcites from Freiburg, Saxony, Fig. 388. When the indi- vidual spherules are small, the mass resembles fish roe, or oolitic. FIG. 388. Oolitic Calcite. Carlsbad, Bohemia. 272 MINERALOGY FIG. 389. Botryoidal Prehnite. Bergen Hill, New Jersey FIG. 390. - Stalactite of Calcite. Copper Queen Mine, Bisbee, Arizona. PHYSICAL PROPERTIES 273 Botryoidal. When in rounded irregular masses, usually with a smooth surface and an internal radiated structure, as prehnite, Fig. 389. When in forms resembling kidneys, Often termed reni- form, as some hematites. Stalactitic. Minerals are often formed by the evaporation of solutions dripping from the walls and ceilings of caves ; the solids are deposited in the form of icicles or stalactites, as calcite, mala- chite, or limonites, Fig. 390. Stalactites are often long, slender, and hollow, the solution flowing down the hollow tube and deposit- FIG. 391. Stalagmite of Calcite. ing the mineral substance only on evaporating at the lower end. Such stalactites are strawlike. Stalagmites. Are the reverse of stalactites, or the structure which forms on the floor of caves under the drip from a stalactite, Fig. 391. Mammillary. When the crusts are composed of rounded masses or nipple-like structures. Other terms in common usage in the description of minerals will need no particular explanation. Color of minerals. The outward appearance of minerals, due to other causes than form or structure, is caused by light, or color and luster. The color of a specimen when opaque is caused by re- flected light wholly; when transparent, to transmitted light in combination with reflected light. When white light falls upon a mineral, some rays are absorbed and the complementary color is 274 MINERALOGY reflected as the color of the surface, while the depth or tone is modified by the physical condition of the surface, for a highly pol- ished surface will glisten with the quantity of light reflected, and a rough, earthy, or powdery surface will appear dull from the amount of light diffused. Combined with these surface effects, in trans- parent minerals, is the color of transmitted light, characteristic in some cases, as in crocoite, of the compound, in others, as in quartz, due entirely to impurities. Since the nature of the surface is variable, it has been found more accurate in comparing the color of mineral specimens to use the very finely ground powder, which insures a surface always of the same nature, from which the same amount of light will be reflected or diffused, and the color will not depend upon fortuitous causes. The most convenient method of obtaining the fine powder quickly is to draw, with a firm pressure, the rounded corner of the specimen quickly across an unglazed porcelain surface, when most minerals will leave a mark, termed the streak, the color of which is that of the fine powder. The color of the streak is of great help in the rapid determination of minerals. The streak is little affected by the structure of the specimen, as hematite occurs in well-formed crystals, micaceous, compact, massive, and earthy, with a blood-red color, gray, steel-gray to nearly black, iridescent, or brown; all of these varieties will yield a cherry-red streak. A mineral is described as of metallic luster when the streak is dark in color and the specimen is opaque on the thin edges or in thin sections and has a shiny surface, common to most metals, as tin or iron. When the streak is dark and the surface is not shiny, it is of submetallic luster. If the streak is light in color or the thin edges and sections trans- mit light, it is non-metallic in luster. The color of metallic and submetallic minerals is more character- istic and less variable than in non-metallic. It is therefore more reliable as a means of identification. The true color of a mineral is shown only on a freshly broken surface, as by oxidation or weathering the natural surface may be entirely changed. The beautiful iridescence of some pyrites, chalcopyrite, limonite, and hematite, is caused by a thin film of oxides or hydroxides which yield spectral colors through the inter- ference of light. The surface change in color due to chemical change is termed PHYSICAL PROPERTIES 275 tarnish. Non-metallic minerals are transparent on thin edges or in thin sections, such as rock sections, which are under .04 mm. Many minerals which are opaque in coarse fragments, as the black tourmalines or rutile, will be transparent in thin sections; even metals, as gold, will transmit light if the sheets are thin enough. The opacity to light is relative, depending upon the thickness of the section. A mineral is said to be transparent if the outline of objects can be distinguished through it; translucent when the light is diffused and the outlines of objects are no longer distin- guishable through the specimen. Some effects are due to both diffusion and interference of light, as opalescence, and the milki- ness of some quartz, the play of colors in the fire opal, or the color yielded by some labradorite when viewed at certain angles with the twinning planes. The star sapphires and cat's-eye are due to reflections and a fibrous structure. The prism colors or banded effects produced by films of air in thin cleavage cracks are due to interference. The variability of color in non-metallic minerals is well illus- trated in quartz, a mineral which, when pure, is colorless. Very small amounts of some oxides which are present as impurities act as a pigment, yielding decided colors even when the amount of coloring material is so small as not to be detected by the ordinary chemical tests. Certain colors are characteristic of chemical elements, as most copper minerals are blue, green, or red; and copper compounds will yield these colors when present in other minerals as impurities or inclusions. Calcite, smithsonite, and quartz are_ often colored green by copper compounds. Chromium yields a green color, as in the green of some garnets and in the emerald. The red of the ruby as well as the red of crocoite is chromium in a different form or ion. Nickel also yields a green color, as in chrysoprase, a variety of quartz. Iron as an impurity will yield shades of red, brown, yellow, green, or blue according to its state of oxidation or combi- nation. The yellow calcites of Joplin, Missouri, contain ferrous iron carbonate and the red jaspers and bloodstones are colored by ferric iron. Very small quantities of manganese yield intense colors; pinks, amethystine, and some greens are caused by manganese. The rqse color of some quartz and tourmaline is caused by titanium, which also when in a lower state of oxidation will yield a blue. Cobalt is the most intense of all mineral pigments and many blue minerals owe their color to cobalt ; when in cobaltic form, the ion is 276 MINERALOGY pink, as in cobalt bloom. Uranium minerals are yellow or green. Molybdenum yields greens and yellows, and tungsten yellow or blue; while the browns and yellows of most quartz, as smoky quartz, are caused possibly by organic matter, and the delicate blues, violets, greens, and yellows of fluorite, apatite, barite, and topazes are also attributed to organic matter. Inclusions of colored minerals within the body of a colorless min- eral are often the cause of color in minerals, as the red or flesh- FIG. 392. Section of Tourmaline, with an Uneven Distribution of Color. Brazil. colored orthoclases of some granites is caused by included scales of hematite, as is also the peculiar appearance of aventurine quartz. The color of minerals caused by pigments is not always evenly distributed, as during the period of growth of some crystals the chemical composition of the mother solution may have changed as regards these minor constituents, resulting in the change of color of the forming crystals, producing phantoms or outlined crystals within the body of larger individuals, as the phantom amethysts of Schemnitz, Hungary, and the phantom fluorides of Cumberland, England. The best illustration of the uneven or irregular distribu- tion of color is the variegated tourmaline of many localities, as of Haddam, Connecticut ; Pala, California, or Brazil. In the latter the colors appear in concentric bands as illustrated in the section, PHYSICAL PROPERTIES 277 Fig. 392, at right angles to the vertical axis of a tourmaline from Brazil. In the center there is an area of concentric bands of differ- ent shades of pink, then a colorless band, outside of which is a band of green. Again the colors may be distributed lengthwise the crystal, as many specimens from Mesa Grande, California, in which one end may be green, the other pink, and often separated by a colorless middle band across the body of the crystal. The delicate coloring of some transparent crystals is often changed by heating ; in this way the pink topazes are produced by heating the originally yellow stones from Brazil. This process is known as " pinking," and if the heat is too great, the crystal becomes colorless. In the description of colors in minerals Werner, nearly two hun- dred years ago, fixed the following eight colors as primary, and since that time mineralogists have been accustomed to describe minerals in terms of these eight colors with their variations as follows : WHITE, either clear, transparent, or translucent Snow-white, as Carrara marble. Greenish white, as talc. Reddish white, as some calcite. Bluish white, as some calcite. Yellowish white, as some calcite. Milk-white, as some quartz. Grayish white, as some calcite. GRAY Blue-gray, as some wernerite. Yellowish gray, as some dolo- Pearl-gray, as some dolomites. mites. Smoke-gray, as flint. Ash-gray, as leticite. Greenish gray, as tremolite. BLACK Grayish black, as ilvaite. Brownish black, as allanite. Velvet-black, as black tourma- Reddish black, as acmite. line. Bluish black, as tourmaline (in- Greenish black, as augite. dicolite). BLUE Blackish blue, as dark azurite. Prussian blue, as cyanite. Azure-blue, as light azurite. Smalt-blue, as dumortierite. Violet-blue, as some fluorites. Indigo-blue, as some vivianites. Lavender-blue, as some sodalites. Sky-blue, as turquoise. 278 MINERALOGY GREEN Verdigris-green, as amazon stone. Celandine-green, as some beryl. Mountain-green, as light beryl. Leek-green, as prase. Emerald-green, as emerald. Apple-green, as chrysoprase. Olive-green, as olivine. Grass-green, as some diallage. Pistachio-green, as some epidote. Black-green, as some serpentine. Oil-green, as some beryl. Siskin-green, as torberniteo YELLOW Sulphur-yellow, as sulphur. Straw-yellow, as yellow topaz. Wax-yellow, as orpiment. Honey-yellow, as some blende sphalerite. Lemon-yellow as sulphur. Ocher-yellow, as some limonites. Wine-yellow, as topaz. Cream-yellow, as some kaolinite. Orange-yellow, as some orpi- ment. Aurora-red, as realgar. Hyacinth-red, as crocoite. Brick-red, as some jasper. Scarlet-red, as, cinnabar. Blood-red, as pyrope. RED Carmine-red, as ruby. Rose-red, as rose quartz. Crimson-red, as ruby. Peachbloom-red, as erythrite. Flesh-red, as some feldspars. BROWN Reddish-brown, as zircon. Pinchbeck-brown, bronzite. Clove-brown, as axinite. Wood-brown, as some asbestos. Hair-brown, as some wood opals. Liver-brown, as some jaspers. Chestnut-brown, as some hema- Blackish brown, as some tites. chromites. Luster. Luster is a term used to describe or denote the peculiar character of light reflected from the surfaces of minerals. The differ- ence in quality of reflected light is caused not only by the character of the surface, but also by the structure and index of refraction of the specimen. All faces of the same crystal form, as the cube faces or cubical cleavage surfaces of galena, will have the same luster, while the three pinacoids are at right angles to each other, as the three directions of the cubical cleavage, but each of the pinacoids may have a distinguishing luster. PHYSICAL PROPERTIES 279 In addition to metallic, submetallic, or non-metallic lusters, which are in a large measure dependent upon opacity to light, the follow- ing terms are used : Adamantine luster is a high, shiny, and brilliant luster, usually connected with minerals with a high specific gravity and index of refraction. It also gives the impression of being very hard. Good examples are cuprite, rutile, cassiterite, sphalerite, cerussite, and diamond. Vitreous or glassy luster, as of broken glass, bright and shining, like quartz, apatite, beryl, and most of the silicates. Greasy or resinous is a vitreous luster as if oiled or like resins, as serpentine. Waxy, very much like resinous, like calcedony. Pearly luster is well shown in mother-of-pearl, due to a combina- tion of surface reflection and a shelly structure, as in brucite, talc, and the basal cleavage of apophyllite and the pinacoidal cleavage of heulandite. Silky luster is the luster of satin, due to a fibrous structure, as in satin spar, asbestos, and enstatite. Dull, as in chalk or kaolinite, is where the reflected light is diffused. Phosphorescence. Some minerals and chemical compounds possess the property of transforming energy of other forms into light and continue to emit a characteristic glow long after the exciting agent or cause has been removed or ceased to act. Calcium sulphide mixed with small amounts of bismuth is used as a luminous paint and will continue to glow for hours after exposure to sunlight. The hexagonal zinc blende, wurtzite, will glow under the emissions of radium. Diamonds, willemite, and kunzite phosphoresce when exposed to the Rongten ray or ultra-violet light. In the case of willemite the ultra-violet light is used to test the completeness of the mechanical separation from the gangue and other zinc min- erals, as every remaining particle of willemite will glow brilliantly when exposed to ultra-violet light. Other minerals, as quartz, become luminous by friction, or when fractured, as some micas. Specimens of the same species may vary greatly in their power to phosphoresce and it would seem not to be a property of pure chemical compounds or pure minerals but is caused in most cases by impurities and is often restricted to local- ities, as all the minerals from Borax Lake, California, phosphoresce under ultra-violet light, which is probably due to some common constituent. 280 MINERALOGY Phosphorescence is caused by the lengthening of the wave of the absorbed energy, as the ultra-violet wave and the wave of the Rontgen ray are too short to be detected by the eye, but when lengthened to .00039 mm. affect the eye as light. Fluorescence is much like phosphorescence, only the phenomenon continues during the actual exposure only, as in the barium plati- nocyanide screen, used to visualize the Rontgen rays. Some white fluorites fluoresce in the sunlight with a bluish, milky, or hazy light. This is a property possessed by uranium compounds and some compounds of boron. Uranium nitrate is used in the manu- facture of fluorescent glass. CHAPTER IV THE NATIVE ELEMENTS DIAMOND Diamond. Carbon, C; Isometric; Type, Ditesseral Polar; Common form, o (111); Twinning planes, 111 and 100; Cleavage octahedral, perfect; Brittle; Fracture, conchoidal; H. = 10; G. = 3.51 3.52; Color, white, yellow, brown to black, rarely blue or green; Luster, adamantine to slightly greasy; Transparent to opaque; n = 2.42; Dispersion strong =.063. B.B. Infusible, insoluble in acids. When heated to a high temperature for a long time it burns slowly, forming CO 2 . Colored stones may change color on heating. General description. Always crystalline, usually simple octa- hedrons or rounded hexoctahedrons which are supplementary twins, combinations of the plus and minus hextetrahedrons, in which the octahedral edge is replaced by a reentrant angle. Simple tetrahedral forms and the cube are rare. Crystal faces are often drusy or covered with triangular etch-figures, due to corro- sion, which is also the cause of the rounded appearance of many diamond crystals. Twins after the spinel law, where the face of the octahedron is the composition plane, are not uncommon. The perfect octahedral cleavage is utilized by the cutters in the rough preparation of the stones for the grinders. While the diamond is the hardest known substance, it is brittle and easily broken or ground to powder, the dust of which is used on the wheels or " skeifs " in grinding and polishing the facets of the cut stone. The inclination of all facets of the brilliant is calculated so that the greatest amount of light is totally reflected and returned. Owing to the high index of refraction, rays which meet the lower facets at an angle greater than 24 13' are inter- nally totally reflected, and emerge above the girdle, owing to the very strong dispersion, yielding prismatic color. The high index of refraction and strong dispersion are the two properties which 281 282 MINERALOGY produce the brilliancy and luster of a well-cut, perfect stone. Diamond is pure carbon, yielding on combustion carbon dioxide and .05 to .20 per cent, of ash. This foreign matter is due to impuri- ties or inclusions, and it is these impurities which cause the various shades of color ; the yellow shades predominate, and while often more brilliant are not as valuable as those of a steel-white color; blue, green, or red diamonds are the most valuable of all gems. Cleavage fragments and dark brown specimens are termed bort and are used in glass cutters, or reduced to dust for polishing. Another most important use is in core drilling, a most convenient and economical method of prospecting mining properties. Car- bonado, a black variety, from the province of Bahia, Brazil, occur- ring in rounded masses, lacks the perfect cleavage of the transpar- ent stones. It is slightly porous and therefore of a lower specific gravity, yet harder than the well-crystallized material, and is said to yield better results in drilling than the diamond fragments. The commercial unit of weight used in estimating the value of diamonds, as well as other precious stones, is the carat. Like all of the old units of measure the carat of merchants of different coun- tries varied from 188.5 to 254.6 milligrams ; the weight commonly used was from 205 to 207 milligrams. The metric carat of 200 milligrams is now the legal carat in all countries using the metric system. All diamonds of the ancients and of Europe until 1727, when diamonds were discovered in Brazil, were from the East, where they were obtained from alluvial washings and in conglomerates, espe- cially at Purteal and Golconda, India. The noted diamonds oi these fields are the Koh-i-noor, of 186 carats and now recut to 106 ; the Pitt or Regent, a yellow stone of 137.5 carats, now in the Galerie d'Apollon in the Louvre, Paris (this stone was appraised in 1791 at 12 million francs); the Orloff, of 194 carats; the Blue Hope, of 44.5; and probably the Great Mogul, of 279 carats. In 1727 diamonds were discovered by the miners in the gold washings of Minas Geraes, Brazil ; since then these workings have yielded continuously large quantities of good stones. Here also the crystals are obtained in river washings and prairie deposits and are associated with a peculiar quartz schist or flexible sand- stone, termed itacolumite. The most famous diamond of the Brazilian field is the "Star of the South," weighing 247.5 carats uncut and 125 when cut. The Vaal River locality of South Africa was discovered in 1867, THE NATIVE ELEMENTS 283 where the first diamond was taken from the sands of the river by some Boer children ; attracted by its brightness, they carried it home to add to their playthings. It weighed in the rough 21.25 carats and sold for 500 pounds. In 1869 the " Star of South Africa " was found by a black shepherd on the Orange River ; this was a magnificent white stone of 83.5 carats and cut to 46.5. After passing through several hands it was sold to the Earl of Dudley for 125,000 dollars. In 1870 diamonds were found at Kimberley, for the first time unmistakably in their primary position, contained in a peculiar peridotite in the form of pipes and plugs, filling the craters of ancient volcanoes. By decomposition this rock forms the famous " blue earth " from which the South African diamonds are obtained, and in which they are associated with garnets, magnet- ite, enstatite, augite, chromite, olivine, corundum, etc. Other similar pipes were subsequently discovered, all of which have been consolidated in the De Beers Company limited, which has produced nearly all the world's supply of diamonds for the last twenty-five years. The largest diamond ever found was the Cullinan, weigh- ing 3253f carats, taken on June 6th, 1905, from the walls of the Premier mine near Pretoria, South Africa. Before this discovery, the " Excelsior Jubilee," weighing 97 If carats, discovered in the Jagersfontein mine in the Orange River colony, was the largest. Both of these diamonds, though they were of beautiful color, owing to internal flaws were cleft and cut into various stones. The Culli- nan was originally purchased by the Transvaal Assembly for 1,000,000 dollars and presented to Edward VII. It is now a part of the Royal Regalia deposited in the Tower of London. In the United States diamonds have been found in North .Carolina, Georgia, Virginia, Colorado, California, and Wisconsin, all of which were loose in gravel or sand. In 1906 diamonds were discovered in Pike County, Arkansas, in a peridotite resembling, in many respects, the deposits of South Africa. Several companies have been formed and some 1200 diamonds have resulted, yield- ing, when cut, gems of good color. They are small, very few weighing over one carat. The largest yet found was a stone of 6.5 carats. The origin of diamonds has not as yet been satisfactorily ex- plained. They have been crystallized, probably from carbon dis- solved in a fused magma and under high pressure. The source of this dissolved carbon is in doubt ; it may have been brought up from depths with the igneous rock or acquired from shales contain- 284 MINERALOGY ing organic matter, through which the magma, while still in a fused condition, was forced. In most cases this magma has been basic, containing large quantities of magnesium silicates. After solidi- fication they are of the nature of peridotites. Artificial diamonds were produced by Moissan, by dissolving carbon in fused iron; upon cooling the fusion quickly in melted lead, the outer portions solidify first and the resulting contraction subjects the still liquid interior to enormous pressure. Under these conditions the excess carbon was separated as small diamonds. If the fusion was cooled slowly and without pressure, the more stable crystalline form of carbon, graphite, was formed. The diamonds contained in meteors, as the Canon Diablo specimens, must be of this nature. Diamonds were also produced by J. Friedlander, who dissolved graphite in fused olivine ; from these experiments it was found that fused magnesium and calcium silicates dis- solved carbon and favored its separation on cooling in the form of diamond, a condition very similar to that of the natural deposits. GRAPHITE Graphite. Carbon, Black Lead, Plumbago ; C ; Hexagonal ; Type, Dihexagonal Alternating; c = 1.3859; J)001 A 10ll = 58, r A r' = 94 31' ; Common forms, c (0001), r (lOfl) ; Cleavage basal, perfect ; Laminae, pliable ; H. = 1-2 ; G. = 2-2.3 ; Color, black to steel-gray; Streak, gray to black; Luster, metallic to dull and earthy ; Opaque, feels greasy and marks paper. B.B. Infusible, deflagrates on coal when mixed with nitre and heated to a high temperature. Insoluble in acids. General description. Occurs when crystalline in thin tabular crystals flattened parallel to the base, often in foliated masses, radiated scaly, compact or earthy. Many specimens are impure from oxides of iron, clay, or mixture with sand. Cliftonite is an isometric form, harder than graphite, contained in a meteoric iron from Australia and also from Cooke County, Tennessee ; these crystals have been regarded as pseudomorphs after diamond. Graphite is very common in crystalline schists, as in the Adiron- dacks and at Ticonderoga, New York, and High Bridge, New Jersey, where it has been formed from the organic matter, contained in the original sedimentary deposits, by the metamorphic action of heat THE NATIVE ELEMENTS 285 and pressure. It occurs in disseminated scales in crystalline lime- stones, as the Laurentians of Canada. The graphite of Colfax County, New Mexico, was formed by contact of intruded igneous rock with coal beds. Graphite occurs associated with diamond in the Canon Diablo meteor, and with native iron in the basalt of Ovifak, Greenland. A pegmatite of Maine contains 9 per cent, of graphite, which has separated from the magma after the feld- spars and is contained as inclusions in the quartz. Graphite con- tained in fissures and veins at Ticonderoga suggests a pneumato- lytic origin. Commercially most of the graphite mining in the United States is carried on at Ticonderoga ; this is the crystalline form of graph- ite and is used in the manufacture of crucibles, for the melting of alloys and the refining of metals. The Ceylon product is consid- ered better, being more fibrous, requiring less binder ; at the present time these mines supply most of the world's product. Graph- ite has been used in pencils for several hundred years ; an amor- phous product from Sonora, Mexico, is considered to be the best for this purpose. A* finely ground graphite is mixed with oil as a lubricant. It is also used for electrodes in the electrochemical industries, and the impure forms are used as a paint to protect iron- work from rusting. Artificial. Fused metals dissolve carbon, iron as much as four per cent., a large portion of which separates as graphite on cooling. Thus cast iron contains carbon as graphite, which has been formed under the same conditions as that contained in meteoric iron. It is common in slags of blast furnaces. Graphite is produced at Niagara Falls electrolytically, by the Acheson process, competing commercially with the natural product. SULPHUR Sulphur. S ; Orthorhombic ; Type, Digonal Holoaxial ; a : b : c = 0.8131 : 1 : 1.9034 ; 100 A 110 = 39 6', 001 A 101 = 66 52', 001 A Oil = 62 17', 111 A 001 = 71 39'; Common forms, c (001), P(lll),p (111), e (101), s (113), n (Oil) ; Twinning plane, 101 ; Cleavage, c and m perfect, p imperfect ; Brittle ; Fracture, conchoi- dal; H.= 1.5-2.5; G.= 2.05-2.09; Color, sulphur yellow and va- rious shades of yellow ; Streak, white ; Luster, resinous ; Trans- parent to translucent; a = 1.950, p = 2.038, -y = 2.240 ; y- a = .290; Optically (+) ; Plane of the optic axes = 010; Bx a = c, 2V = 69.5. 286 MINERALOGY B.B. Fuses easily at 114.5 and burns with a blue flame, form- ing sulphur dioxide. When pure, volatilizes entirely. Insoluble in acids. Dissolves in carbon disulphide. General description. Crystals are pyramidal in habit, termi- nated by the base, the two domes e and n, and the pyramids s ; many other forms have been described, all of which are rare. Some FIG. 393. Sulphur Crystals from Girgenti, Sicily. crystals are sphenoidal in habit, indicating the holoaxial symmetry. The best examples of sulphur crystals are found at Girgenti, Sicily, where they occur associated with celestite and other sulphates. More often the crystals are small or the sulphur is incrusted, massive, or powdery, mixed with clay, marl, or other impurities. Sulphur is a non-conductor of heat, and a peculiar crackling noise may be noted when a crystal is held to the ear, in the hand, due to the un- even heating ; in this way crystals often fall to pieces. Sulphur is deposited around volcanoes and solfataras, where it is condensed from vapors or reduced by the interaction of S0 2 and H 2 S, or again by the oxidation of H 2 S. Many hot springs con- tain H 2 S in solution which on oxidation deposits sulphur. In sedi- mentary deposits sulphur is formed in the reduction of sulphates, and is often associated therefore with celestite and gypsum. It has also been observed in the cracks of galena, as at the Wheatley mine, Pennsylvania. Deposits of sulphur in the United States are found in the Yellow- stone Park, in a rhyolitic tuff ; at Black Rock, Utah ; at Cody, THE NATIVE ELEMENTS 287 Wyoming. But by far the most important deposit, is at -Bayou Choupique, Lake Charles, Louisiana, where a bed of almost pure sulphur 100 feet thick, lies at a depth of 440 feet below the sur- face. This deposit furnishes nearly all the 350,000 tons annually consumed in the United States, most of which was formerly imported from Sicily. Large quantities of sulphur are used in the wood pulp industry ; in the manufacture of matches ; in blasting powder ; in vulcanizing rubber, and in bleaching through the chemical action of SO 2 . Artificial crystals may be formed by evaporating a saturated solution of sulphur in carbon disulphide. There are many allo- tropic forms of sulphur ; a-sulphur is stable at ordinary tempera- tures, while monoclinic, /3-sulphur, forms when melted sulphur is allowed to cool until a crust forms, which is broken and the still liquid interior is poured off, when crystals of this monoclinic form will cover the walls. On standing they become opaque from the formation of small crystals of the more stable orthorhombic form. PLATINUM Platinum. Pt ; Isometric ; Type, Ditesseral Central ; Common forms, c (100), o (111), d(110); Malleable; Sectile and ductile; H. = 4-4.5; G. = 14-19, when pure, 21.42; Color and streak, steel-gray; Luster, metallic; Opaque. B.B. Infusible, fusing point 1755. Soluble in hot aqua regia. For other tests see page 582. General description. Crystals are not common, but the cube, octahedron, and rhombic dodecahedron occur. Usually it occurs as fine grains or in irregular rolled masses. Native platinum is always alloyed with other metals of the platinum group, as osmium, iridium, paladium, rhodium, and ruthenium, all of which occur only in the native metallic state, with the exception of plati- num, which occurs in sperrylite (PtAs 2 ) as an arsenide, and ruthe- nium in laurite (RuS 2 ) as a sulphide. In addition platinum often contains iron, nickel, and gold, to which its variable specific gravity and hardness are due. Deposits of platinum are associated with basic rocks, as serpen- tine and peridotite. Its most constant companion is chromite. Platinum was first discovered in the gold washings of the Pinto River, Colombia, South America, about the year 1720, and in the 288 MINERALOGY alluvial deposits of the Ural Mountains, Russia, in 1822. Here nuggets weighing as much as. 18 kilos were found. The largest, weighing 18.57 kilos, is in the Demidoff collection of minerals at St. Petersburg. The larger part of the world's supply of the present day, about 15,000 pounds annually, is derived from the Russian de- posits. A peculiar black sand left with the gold in the washings of the Pacific slope, particularly in British Columbia and the states of Oregon and Washington, contains small amounts of platinum; from this source a few ounces are obtained annually. Platinum has been reported as contained in a serpentine of the Urals ; in a pegmatite of Copper Mountain, British Columbia; in a decomposed schist of Broken Hill, Australia; in limonite nodules in Mexico; in an altered limestone of Sumatra. In addition it is connected with certain sul- phides, as the pyrrhotite of Sudbury, Canada ; covellite in Wyo- ming; and chalcopyrite of the Key West mine near Bunker vi lie, Nevada, where it is associated with nickel, as at the Sudbury locality. Possibly in these associations platinum may be in the form of arsenide, as sperrylite has been reported from both Sud- bury and the British Columbia localities, but has not as yet been reported from Nevada. Owing to the high fusing point and its insolubility in single acids, platinum crucibles are used in chemical analyses. It is also used in thermoelectric couples for the measurement of high tempera- tures ; as a catalyzer to oxidize SO 2 to S0 3 in sulphuric acid works. Having the same coefficient of expansion as glass, it is used to carry the electric current through the glass walls of physical apparatus. There are many other minor uses ; and since the supply cannot keep pace with the demand, the price is constantly increasing, until at the present time platinum is more than double the value of gold. COPPER Copper. Native Copper, Cu; Isometric; Type, Ditesseral Central ; Common forms, a (001), o (111), d (101), h (410) ; Twin- ning plane, 111 ; Malleable, ductile; Fracture, hackly; H. = 2.5-3 ; G. = 8.8-8.9; Color, copper red; Streak, shining; Metallic; Opaque. B.B. Easily fusible (1084). In the blue cone of the 0. F. yields a green flame. On coal becomes black after fusion from the formation of black oxide. Dissolves in HNO 3 or HC1, yielding a solution which becomes intensely blue on the addition of an excess of ammonia. THE NATIVE ELEMENTS 289 General description. Copper crystallizes in cubes, octahedrons, and tetrahexahedrons; other forms are rare. Twins after the spinel law are not uncommon. It occurs more often in distorted forms, or in arborescent, reticular, dendritic, filiform, and irregular masses. Pseudomorphs after other copper minerals, as cuprite, malachite, FIG. 394. Native Copper and Quartz from Lake Superior. and azurite, are often formed by double decomposition and reduc- tion. Native copper is usually coated with a coat of oxides or car- bonates ; many masses of cuprite still contain as a central nucleus some of the metallic copper from which they were formed. Native copper occurs as a secondary product, either formed by precipitation from solution, or by the chemical action of reducing agents upon minerals containing copper. Percolating ground waters dissolve copper and especially under heat and pressure ; even distilled water will dissolve copper under these conditions, leaching out the original copper content of the igneous rocks and trans- porting it to points, as veins and cavities, where it may be pre- cipitated by contact or by intermingling with other solutions con- taining a reducing agent, as ferrous iron, which is capable, either as oxide, sulphate, silicate, or carbonate, of precipitating copper from its solutions. The large deposits of the Lake Superior region have probably been formed in this way. Here metallic copper, generally in small particles, is contained in the amygdaloid cavities of a conglomerate. One mass of 400 tons was found in the Minnesota mine. Origi- nally the copper must have been contained in the adjacent igneous 290 MINERALOGY rocks, but is now concentrated by solution and precipitation in the conglomerate. Native copper is also found in the Copper Queen mine, Arizona ; in sheets at Enid, Oklahoma, and associated with fossil bones in Peru, evidently reduced by the organic matter of the bone. Small amounts of copper are present, usually as a secondary product, in nearly every copper region and mine; but only in the Lake Superior region does it constitute nearly all the copper content of the ore. The United States produced, in 1911, 550,645 tons, nearly one half of the world's product ; of this Lake Superior region contrib- uted 110.700 tons; Montana, 141,250 tons, and Arizona, 110,500 tons. SILVER Silver. Native Silver, Ag; Isometric; Type, Ditesseral Cen- tral; Common forms, a (100), o(lll), d (101) ; Twinning plane, 111; Malleable and ductile; Fracture, hackly; H. = 2.5-3 ; G. = 10.1-11.1 ; Color and streak, silver- white ; Luster, metallic; Opaque. B.B. Fuses easily (955), and in O. F. on coal yields a brown coat of silver oxide. Soluble in nitric acid. Other tests for silver are given on page 578. General description. Crystals are usually elongated or dis- torted octahedrons. All seven forms of the type occur on silver crystals, but others than the octahedron and cube are comparatively rare. In occurrence it is more often, in dendritic or arborescent growths, due to twinning and parallel positions ; sheets, wire, and disseminated scales are also common. On exposure the bright surfaces become brown to black, from the formation of sulphides. Owing to its solubility and the easily formed sulphides, silver is not found in placer deposits, but it is associated with gold and copper in vein deposits, where most of the native silver is of secondary origin, being reduced from sulphides, chlorides, and other compounds. Masses of native silver weighing from 500 to 1000 pounds have been taken from the deposits of Cobalt, Ontario, where it is asso- ciated with ores of cobalt and nickel. Masses weighing as much as 800 pounds have been found at Huantaya, southern Peru, while beautiful specimens of crystalline and wire silver are obtained in the mines of Batopilas, Mexico ; and Kongsberg, Norway. In the THE NATIVE ELEMENTS 291 United States alloys of silver and copper are found in the copper mines of the Lake Superior region, the Poor Man's Lode, Idaho, and many localities through the West. Mexico is the largest producer of silver, with 72,000,000 l ounces, while the United States produced 57,796,117 ounces, and Canada, 31,500,000 ounces, in 1911. GOLD Gold. Native gold, Au ; Isometric ; Type, Ditesseral Cen- tral ; Common forms, o(lll), d(110); Malleable and ductile; Fracture, hackly ; H. = 2.5-3 ; G. = 15.8-19.3; Color and streak, gold-yellow; Luster, metallic; Opaque. B.B. Fuses easily (1065). Insoluble in acids except aqua regia, also in nascent chlorine, and in potassium cyanide in pres- ence of oxygen. For other tests see page 582. General description. When crystalline, usually in octahedrons elongated parallel to one axis or flattened parallel to a face. Also in arborescent and reticular shapes, or sheets, dissemi- nated scales, and rolled water- worn masses, known as nuggets. The purest native gold is said to be that of Mount Morgan, Queensland, 99.7 per cent. gold. Most gold contains silver; electrum FIG. 395. - Crystals of Gold . Australia. from Hungary is a pale yel- low natural alloy containing 30 per cent, silver. Gold may also contain copper, iron, lead, bismuth, platinum, or mercury, most of which reduce its specific gravity and modify its color. Gold is nearly world- wide in occurrence, though in very small quantities except where it has been concentrated by secondary causes. Most of the world's gold, until recent years, was recovered from river sand and alluvial deposits, where, owing to its high specific gravity and insolubility, it has remained behind unchanged for centuries. Such gold is recovered by the simplest of all min- ing, placer, which consists of washing the lighter sands away, in sluices or pans. All placers of the world are worked by modifica- 1 1910. 292 MINERALOGY tions of this simple method, which reduces the cost of handling the enormous quantities of material made necessary. By the hydraulic method in California the cost has been so reduced that a cubic yard of earth can be handled at a cost of two cents. The largest nuggets have been taken from the gravels of Australia, one weighing 190 pounds, another 180 pounds. Gold occurs in quartz veins, especially the veins of schists, porphyry, and those rocks high in silica. Less often do the veins of basic rocks contain gold, though it may be found in some limestones and slates. It is probably more soluble in magmas high in silica. Gold as a primary mineral has been reported in granite from Mexico ; in pitchstone from Chili ; and small amounts of gold are shown by assay to be contained in granites, syenite, basalt, and diabases of California. Gold is soluble in sodium or potassium silicates, ferrous sul- phates and chloride, and the alkali sulphides. The small amounts of gold contained in the country rocks are dissolved by these natural solvents and transported in solution by the percolating waters to veins or cavities where, through a changed physical condition, as a reduction of temperature and pressure, or by contact with a pre- cipitant, they are deposited. As evidence of such concentration, gold is being deposited with the siliceous sinter at the Steamboat Springs of Nevada and at the hot springs of New Zealand, and its presence in sea water has been repeatedly verified. Gold in solution is very unstable and is easily precipitated by such agents as organic matter, ferrous salts, metallic sulphides, especially those of iron, copper, zinc, arsenic, and antimony; with the ores of these metals gold is often associated. Pyrite is a constant companion of gold in quartz veins, inclosing it within the body of the crystals as inclusions ; the pyrite on oxida- tion is mostly carried away, leaving the gold behind, contained in a porous, rusty quartz, always so pleasing in appearance to the old prospector. The largest producing gold mines of the world are on the Rand in South Africa ; at Victoria, Australia ; and in the United States. The United States produced, in 1911, $96,233,428 of pure gold, the mint value of which is $20.6718 per ounce troy or $0.6646 per gram. To this production twenty states contributed, of which Colorado, Alaska, California, Nevada, and South Dakota, in order, were the largest producers. Gold is the basis of the world's coinage; that of the United THE NATIVE ELEMENTS 293 States is 9 parts gold to 1 of copper, or 900 fine. The proportion in jewelry is designated by the carat, 24 carat being pure gold. In England the usual standard is 22 carat, or 916.67 fine. MERCURY Mercury. Quicksilver, Hg; Isometric; Type, Ditesseral Cen- tral ; Crystals, octahedrons ; Liquid at ordinary temperatures ; Solid at 39 with cubic cleavage ; G. = 13.6 ; Color, tin- white ; Brilliant metallic luster; Opaque. B.B. Volatilizes entirely when pure, yielding a gray coat on coal. See tests on page 579. General description. Native mercury is not of common occur- rence. It is found as small metallic liquid globules in the gangue associated with cinnabar, from which it has probably been reduced by organic agents. It also occurs in shales, slate, and marls, as at Idria, Austria, one of the important European localities, where it occurs in a clay slate. The deposits of California and Texas where native mercury is found are also associated with sedimentary rocks, which yield hydrocarbon gases. Hot springs, as the Steam- boat Springs of Nevada, bring mercury to the surface. The mercury of commerce is obtained from the sulphide, cinna- bar. In the United States, California for a long time was the only producing state, but since the discovery of the mercury deposits at Terlingua, Brewster County, Texas has also been a producer. About twenty-one thousand flasks, of seventy-five pounds each, were produced in the United States in 1911, of which California produced seventeen thousand. The greatest demand for mercury is in the amalgamation of silver and gold ores. It is also used in the production of vermilion paint, and in smaller quantities in the sciences and in the construction of electrical apparatus. The metals iron, lead, bismuth, arsenic, and antimony are also found in nature in a free state, but only locally and in very restricted quantities. Iron occurs in meteors and as a primary accessory com- ponent in some basalts, as at Antrim, Ireland, in the trap of New Jersey, and in the dolerites of Mount Washington, New Hampshire. The most noted locality, however, is at Disco Island, West Green- land, where large masses, many tons in weight, have weathered out of the basalt ; these masses were originally supposed to be meteoric. CHAPTER V SULPHIDES, ARSENIDES, ANTIMONIDES REALGAR Realgar. Sulphide of arsenic, As 2 S 2 ; As =^70.1; S = 29.9; Monoclinic; Type, Digonal Equatorial; a: b : c = 1.4403 : 1 : .9729 ; = 66 5' = 001 A 100 ; 100 A 110 = 52 47', 001 A 101 = 40 22', 110 A 110 =105 34', 001 A 011 = 41 39'; Common forms, a (100), b (010), c (001), m (110), r (012) ; Cleavage, b per- fect, c, a, and m less so ; Sectile ; Fracture, conchoidal ; H. = 1.5-2 ; G. = 3.55 ; Color, a dark orange-red, streak somewhat lighter in color ; Luster, resinous ; Transparent to translucent ; Optically (-); Plane of the optic axes, b (010) ; Bx aA c = -f 11; 2V = 92 58'. B.B. On coal in O. F. burns, yielding an arsenic odor and when pure leaves no residue. In the closed tube yields a cherry-red sub- limate of arsenic sulphide, in the open tube yields a white subli- mate of As2O 3 and an S0 2 odor. General description. Crystals prismatic, usually combina- tions of (001), (110), (010) with the prism zone striated lengthwise ; also occurs as compact masses, granular, or in crusts. Usually coated with a yellow film of orpiment into which it changes on ex- posure. Realgar is associated with antimony, arsenic, and silver ores. It occurs as a sublimation product in the lavas of Vesuvius, and at the present time it is being deposited associated with orpiment from the water of the hot springs of Norris Geyser Basin, Yellow- stone Park. In the United States it is mined at Monte Cristo, Washington. Uses. Mixed with lime, realgar is used in tanning, to remove the hair from the hides; it furnishes the white lights in pyro- technics. It is also used as a pigment, but the commercial, or ruby, arsenic is an artificial product. When realgar is dissolved in sodium bicarbonate at 150 under pressure, upon cooling the solution, it separates as crystals. 294 SULPHIDES, ARSENIDES, ANTIMONIDES 295 ORPIMENT Orpiment. Arsenic trisulphide, As 2 S 3 ; As^ = 61, S = 39; Mon- oclinic ; Type, Digonal Equatorial ; a : b : c = .5962 : 1 : .665 ; J3 = 89 19': 110 A 110 = 62 11', 120 A 120 = 96 23'; Common forms, m (110), u (120), o (101) ; Twinning plane, 100; Cleavage, b perfect, laminae flexible, inelastic; Sectile; H. = 1.5-2; G. = 3.4-4.5 ; Color, lemon-yellow ; Streak, pale yellow ; Luster, resi- nous; cleavage surfaces pearly ; Transparent to translucent; Opti- cally ( ) ; Axial plane, 001. B.B. Fuses and volatilizes entirely when pure, yielding an arsenic odor. General description. Crystals are rare, but small crystals are found in the clays at Tajowa, Hungary. It occurs usually in foli- ated or columnar masses with rounded surfaces. It is associated with and may be formed as an efflorescence on realgar. STIBNITE Stibnite. Antimony glance, Sb 2 S 3 ; Antimony trisulphide ; Sb_= 71.4, S = 28.6; Orthorhombic ; Type, Didigonal Equatorial ; a:b: c = .9926: 1: 1.0179; 100 A 110 = 44 47', 001 A 101 = 45 43', 001 A Oil =45 30', 111 A 110 = 34 41'; Common forms, m (110), p (111), b(010); Cleavage, b perfect, a and m imperfect; Slightly sectile and pliable; Fracture, conchoidal; H. = 2;G. = 4.52-4.62; Color, steel-gray, tarnishing to black; Streak, lead- gray, marks paper; Luster, metallic, splendent on fresh surfaces. B.B. Fuses in O. F. at once (630) and on coal yields a white coat of Sb 2 0s, also an odor of sulphur dioxide. When pure, vola- tilizes entirely, in R. F., yielding a yellowish green antimony flame. Soluble in HC1, but in HN0 3 forms a white insoluble Sb 2 5 . General description. Crystals are long prismatic or acicular with striations and furrows parallel to the c axis, usually ter- minated with the unit pyramid. Numerous forms have been de- scribed, especially on crystals from Ichinokawa, Island of Shikoku, Japan, where brilliant crystals nearly two feet in length have been found ; in some cases these are peculiarly twisted around the vertical axis. Often in radiated groups and aggregates of acicular crystals, as at Felsobanya, Hungary, penetrating the tabular crystals of barite with which they are associated. Massive and granular 296 MINERALOGY varieties of stibnite often appear much harder than 2, from the im- purities. Crystals of stibnite are at times coated with white crust formed by oxidation, and at times the entire crystal has undergone FIG. 396. Stibnite from Lyo Island, Japan. oxidation, forming pseudomorphs of cervantite (Sb 2 4 ) after stib- nite, as at Charcas, Mexico. In the United States stibnite occurs at Lovelocks and Humboldt regions, Nevada ; in Iron County, Utah ; and in Buck County, Idaho. Very little is mined, as most of the American antimony is obtained in the smelting of lead ores, which contain antimony in small quantities. The metal is used principally in the alloys, type, babbett, and britannia metal. The trisulphide is used to produce the " Bengal Fire/' the trioxide in the glaze of enameled ironware and in color- ing glass and porcelain yellow. Many of its salts are used in medi- cine. BISMUTHINITE Bismuthinite. Bi 2 S 3 . The trisulphide of bismuth is a rare mineral, occurring in striated, irregularly terminated prisms. It is found in small quantities in Rowan County ^ North Carolina, asso- ciated with gold, and in the gold ores of Goldfield, Nevada. Lead ores contain bismuth and most of the commercial metal is produced in the electrolytic refining of lead. MOLYBDENITE Molybdenite. MoS 2 ; Sulphide of molybdenum; Mo = 60, S = 40 ; Hexagonal ; Type, Dihexagonal Equatorial ; c = 1.098; SULPHIDES, ARSENIDES, ANTIMONIDES 297 0001 A lOlO = 65 35' ; Crystal forms, c (0001), m (lOlO) ; Cleavage, c perfect, laminae flexible but inelastic; H. = 1-1.5; G. = 4.7-4.8; Color, lead-gray to bluish gray to bluish black, marks paper, and feels greasy ; Streak, bluish gray ; Luster, metallic ; Opaque. B.B. Infusible in the forceps, but yields a green flame. The powdered mineral on coal in O. F. yields a white coat. In the open tube yields SO 2 . Decomposed with HNO 2 , moistened with H 2 SO4, and evaporated in a porcelain crucible, yields a blue residue on cool- ing. General description. Crystals are tabular, parallel to the base, and six-sided, with the prism faces irregularly striated and furrowed horizontally. Several prisms have been described, but good faces are rare. Usually much like graphite in appearance. In flat scales disseminated through granite and pegmatites, as at Cooper, Maine. It occurs also in gneiss, schist, gabbro, granular limestones, and in quartz veins, as at Chelan County, Washington, and Beaverhead County, Montana. Crystals two 'to three inches across have been found in Okanogan County, Washington, and at Aldfield, Pontiac County, Quebec. While molybdenite occurs in many localities in the United States, they are all small deposits, two of which are at present mined, that at Cooper, Maine, and in Washington, even though molyb- denite is the principal ore of the metal and is quoted as being valued at $1.50 per pound. The metal is used in making tool steel ; as ammonium molyb- date in the determination and separation of phosphoric acid in iron ores ; in the staining of leather, and as sodium molybdate in the coloring of pottery blue. Artificial crystals of molybdenite have been formed by fusing the oxide with sulphur and potassium car- bonate. ARGENTITE Argentite. Silver glance ; Ag 2 S ; Sulphide of silver ; Ag = 87.1; S = 12.9; Isometric; Type, Ditesseral Central; Common forms, o(lll), a (001), d(110); Twinning plane, 111, interpenetrating ; Cleavage, a and d in traces ; Sectile and malle- able ; H. = 2-2.5 ; G. = 7.2-7.35 ; Color, dark lead-gray ; Streak, gray, shining ; Luster, metallic ; Opaque. B.B. Fuses with intumescence in O. F. on coal, yielding a globule of silver and a sulphur dioxide odor. 298 MINERALOGY General description. Crystals are octahedrons, rhombic dodecahedrons, cubes, or combinations of these forms ; all forms of the type have been observed on argentite, but the other four forms are rare. Fresh surfaces are bright and shiny, but like all silver minerals become dark on exposure. Its occurrence is more often dendritic, granular, or disseminated. At the Comstock Lode, Nevada, it alone ^^^^ constitutes a work- ^HiftjB able silver ore ; also at Port Arthur, Lake Superior. It occurs in small amounts in most silver mines, and in the cobalt region, Canada, in consid- erable quantities. It is usually as- sociated withsteph- anite, galena, py- rite, cobalt, and nickel ores, gold, and silver, the latter being a secondary product reduced from the sulphide. Argentite and galena are isomorphous, and the latter, especially the fine granular varieties, contains small amounts of silver, and the smelting of galena yields each year a considerable amount of the world's pro- duction of silver. Silver in solution, either as the sulphate, carbonate, or nitrate, is precipitated by the natural sulphides as pyrite, chalcopyrite, born- ite, or galena, a reaction which without doubt plays an important part in the secondary enrichment of silver ores. FIG. 397. Argentite from Freiberg, Saxony. GALENA Galenite. Galena ; Lead glance ; PbS, lead sulphide ; Pb = 86.6, S = 13.4; Isometric; Type, Ditesseral Central; Common forms, a (100), o (111), d (110) ; Twinning plane, 111, both contact and interpenetrating ; Cleavage cubic, perfect ; Brittle ; Fracture, subconchoidal ; H. = 2.5-2.75; G. = 7.4-7.6; Color and streak lead-gray; Opaque. SULPHIDES, ARSENIDES, ANTIMONI.DES 299 B.B. Fuses easily on coal in the O. F., yielding SO 2 fumes and a yellow oxide of lead coat, which is often quite white with lead carbonate or sulphate. In R. F., especially when mixed with soda, is reduced to malleable lead. The soda fusion, when placed on a silver surface and moistened, leaves a black stain (S). Dissolves in nitric acid, forming insoluble white lead sulphate (PbSO 4 ). General description. Crystals are usually cubic or combina- tion of the cube and the octahedron, less often the rhombic dodeca- hedron; the simple octahedral habit is rare. Other forms occur FIG. 398. Galena Crystals. Bavaria. which at times give the crystals a rounded appearance; cube faces are often vicinal. Twins after the spinel law art? flattened, as is usual with twins of this class. Cleavage is cubical with brilliant surfaces, and in some cases striated from polysynthetic twin- ning ; in rare exceptions, as at Lancaster, Pennsylvania, and Nord- marken, Sweden, the cleavage is octahedral ; such specimens have always been found to contain bismuth. Massive, granular, and disseminated varieties are common, but fibrous and plumose speci- mens are rare. Galena contains as impurities zinc, copper, cad- mium, bismuth, arsenic, and antimony, possibly as sulphides; also gold and silver; often the silver value is greater than that of lead, when it constitutes a true silver ore. Galena occurs both as a primary and secondary mineral, but by far the most important commercially are the secondary vein de- posits or those filling cavities in limestone formations, where it is associated with sphalerite and chalcopyrite. The gangue of such 300 MINERALOGY veins is usually calcite, siderite, barite, or fluorite. Such deposits have been formed by the precipitation of lead, carried in solution as the carbonate, sulphate, or even the sulphide (as galena is depos- ited by some thermal springs), by water, percolating down from the superficial areas, which tends to dissolve the oxidized ores at the surface and again deposit them as sulphides at lower levels; de- posits formed in this way are apt to become poor at depths. Galena may be deposited by ascending solutions, in which case the supply is brought nearer the surface from depths; also lead sulphide is volatile without decomposition when heated in an atmosphere of many gases, and on cooling recrystallizes as cubes ; galena of this nature has been observed in the lavas of Vesuvius. Galena is very widely distributed ; of the 24 states commercially producing lead ore in 1909, Missouri, Idaho, Utah, and Colorado produced more than three quarters of the 350 thousand tons of that year. The deposits of Missouri, Southern Illinois, Wisconsin, and the Mississippi valley generally are found in limestone and dolomite, Those of Idaho and Colorado are associated with igneous rocks as well as dolomite ; others, associated in veins with gold, silver, and copper ores, are of a complex nature, and the usual gangue mineral is quartz. Artificial galena crystals have been produced by the volatiliza- tion of precipitated sulphide, and by the precipitation of a solution of lead nitrate containing free nitric acid. Octahedral crystals are formed when one part of lead sulphide is fused with six parts each of potash and sulphur. CHALCOCITE Chalcocite. Copper glance ; Cu 2 S; Cuprous sulphide; Cu = 79.8, S = 20.2 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = . 5822:1: .9701; 100 A 110 = 30 12', 001 A 101 = 59 2', 001 A 011=44 8'; Common forms, a (100), b (010), c (001), m(110); Twinning plane, 110 and 032; Cleavage, m distinct; Brittle, frac- ture conchoidal ; H. = 2.5-3 ; G. = 5.5-5.8 ; Color and streak, dark lead-gray, tarnishes to blue on exposure; Luster, metallic; Opaque. B.B. On coal fuses easily and boils with spirting. In 0. F. yields S0 2 fumes and odor. Powdered and roasted without fusing, then heated in R. F. yields malleable copper, also shows copper with the fluxes. SULPHIDES, ARSENIDES, ANTIMONIDES 301 General description. Crystals are tabular parallel to the base, combinations of the base, prism, and pinacoid with a pyramid and dome ; as the prism angle is 119 35', these combinations are pseudo- hexagonal in symmetry. This is even more striking when twinned ; such crystals are found at Bristol, Connecticut, and in Cornwall, England. The base is often striated parallel to the edge c/d. Massive, granular, or disseminated chalcocite is more common than the crystalline. Specimens are often coated with a black crust of melaconite or of the green and blue carbonates, and less often with the blue sulphide, covellite ; all of which are alteration products of chalcocite. Stromeyerite (CuAg) 2 S, as it is variable in composition, is prob- ably a mixture of acanthite, Ag 2 S, the orthorhombic sulphide of silver, and chalcocite, which in many localities contains silver. Chalcocite is a mineral of secondary origin associated with other copper ores, with arsenopyrite, tetrahedrite, sphalerite, pyrite, and galena, in veins, joints, lenses, etc., in which the gangue mineral is principally quartz. At Butte, Montana, probably the largest cop- per camp of the world, the sulphide ore is 50 per cent, chalcocite. It is also an important mineral at Bingham Canon, Utah, asso- ciated with pyrite and chalcopyrite, both of which are enriched with chalcocite. Chalcocite occurs more or less abundantly in all cop- per deposits, in the zone of secondary enrichment, where it has been precipitated from the descending solutions, by contact with pyrite, sphalerite, galena, or sulphides of arsenic, etc. Artificial chalcocite may be produced by heating a solution of cuprous sulphide in a sealed tube with a solution of ammonium sulphocyanate. SPHALERITE Sphalerite. Zinc blende ; black jack, ZnS, zinc sulphide; Zn = 67, S = 33 ; Isometric ; Type, Ditesseral Polar ; Common forms, o (111), d (110), a (100); Twinning plane, 111; Cleavage, dodeca- hedral, perfect ; Brittle, fracture conchoidal ; H. = 3.5-4 ; G. = 3.9-4.1 ; Color, shades of yellow, brown to black, rarely red, green, or white; Streak, pale to colorless; Luster, adamantine; Transparent to opaque ; n. = 2.369 ; Pyroelectric and polar in the direction of the trigonal axes. B.B. Infusible or nearly so ; powdered and reduced on coal with soda yields a white zinc oxide coat, which may be more or 302 MINERALOGY less yellow when cold, from the presence of cadmium ; this coat moistened with cobalt solution and again heated in the O. F. be- comes green ; the soda fusion yields a sulphur reaction on silver. Soluble in hot HC1, evolving sulphuretted hydrogen. General description. Crystals are usually combinations of the plus and minus tetrahedrons with the cube or the dodecahedron, less often with the trigonal trisoctahedron (311). They are often FIG. 399. Sphalerite and Calcite. Joplin, Missouri. rounded on the tetrahedral edges and striated parallel to the inter- section of the two tetrahedrons ; on the cube face these striations are diagonal, showing its hemihedral symmetry. Twins after the spinel law are common and may be developed polysynthetically. Granular, compact, fibrous, or foliated varie- ties are common. Pure sphalerite is white and occurs at Frank- lin, New Jersey, and at Nordmarken, Sweden. It is usually colored with iron or manganese, even though these metals are present only in very small quantities. Cadmium sulphide, being isomorphous with zinc sulphide, is usually present, in some localities as high as 5 per cent. The very dark specimens contain small quantities of indium, gallium, or thallium ; gallium was first discovered by Lecoq de Boisbaudran in 1875 in a specimen of blende from Pierrefitte, Pyrenees. Indium was discovered in 1863 by Reich and Richter SULPHIDES, ARSENIDES, ANTIMONIDES 303 in a blende from Freiberg, Saxony. Specimens containing these rare elements are very dark in color, as that from Roxbury, Con- necticut. Sphalerite is found under the same conditions and in the same formations as galena, which is its constant companion, with the exception that zinc sulphide is more soluble than lead sulphide and in many cases in the oxidized zones the zinc has been carried away in solution, leaving the galena. The oxidation products of sphalerite are calamine and smithsonite, which are ores of the su- perficial areas of zinc deposits. In weathering these may be redis- solved and carried down by the percolating waters. This is well substantiated by the analyses of the mine waters at Freiberg, where it was estimated that the discharge in the valley carried 479 kilograms of zinc per day, or 175,024 kilograms per year. In undisturbed deposits such solutions of zinc are reprecipitated either by pyrite, marcasite, or organic matter as a sulphide, or by replace- ment in limestones as a carbonate. When sphalerite is formed, these reactions take place at low temperatures, as wurtzite, the hexagonal zinc sulphide, is the stable form at high temperature. Simple crystals of sphalerite occur in the dolomites of the Bin- nenthal, and beautiful specimens are obtained at Santander, Spain. In the United States sphalerite is widely distributed in the lime- stones of the Mississippi valley, and the regions of contact of lime- stones with igneous rocks ; all deposits of sphalerite mined for the zinc alone are of these characters. Southern Missouri and the Leadville district, Colorado, both blende deposits, produced 175 thousand of the 280 thousand tons of ore mined in 1909. Sphaler- ite also occurs, but of minor importance, in the metalliferous veins of rocks of all ages. Zinc is used in innumerable ways ; as a metal it is a component of brass, white metal, and german silver. It is used in roofing in galvanizing iron to prevent rusting, and in the zinc boxes as a reduc- ing agent to precipitate gold from the solutions in the cyanide pro- cess; as an oxide in paint, and at the present time is to a large extent displacing white lead. Artificial sphalerite is formed when a solution of zinc is heated in a sealed tube with hydrogen sulphide, at higher temperatures. In fusions, as of precipitated zinc sul- phide with potassium carbonate and sulphur, or with calcium fluo- ride and barium sulphate, the hexagonal form of ZnS, wurtzite, is formed. Sphalerite when heated to bright redness is changed to wurtzite. 304 MINERALOGY ALABANDITE Alabandite. MnS; Manganese sulphide; Mn = 63.1, S = 36.9 ; Isometric ; Type, Ditesseral Polar ; Common forms, a (100), d (110), o (111) ; Twinning plane, 111 ; Cleavage, cubic, perfect; H. = 3.5-4 ; G. = 3.95-4.04 ; Brittle, fracture uneven ; Color, iron-black; Streak, green; Luster, dull submetallic. B.B. On coal in 0. F. yields sulphur dioxide odor; the black oxide remaining reacts for manganese with the fluxes. Soluble in dilute hot HC1, yielding hydrogen sulphide. General description. Crystals are combinations of the cube and the rhombic dodecahedron, often repeatedly twinned after the spinel law ; more often massive or granular. On exposure weathers to a brown color. * Alabandite is not a common sulphide, and for that reason it is commercially unimportant. In the United States it occurs in the Snake River region, Colorado, where it is associated in veins with argentite, pyrite, galena, and rhodochrosite, also at Tombstone, Arizona, in large but rough twinned cubes. CINNABAR Cinnabar. HgS ; Mercuric Sulphide; Hg = 86.2, S = 13.8; Hexagonal; Type, Trigonal Holoaxial; c = 1.1453; 0001 A 1011 = 525_4 / , r v r' = 8723'; Common forms, c (0001), m (1010), r(1011); Twinning axis c, interpenetrating; Cleavage, m perfect; H. = 2-2.5 ; G. = 8-8.2 ; Slightly sectile, fracture conchoidal ; Color, cochineal-red to dark red ; Streak, scarlet ; Luster, adaman- tine; Transparent to opaque; B.B. The same as zinkenite. i General description. Crystals very rare, usually fibrous, plumose, granular, or compact. Often with yellow spots of oxide of antimony on the surface. 322 MINERALOGY Occurs at Pribram, Bohemia ; in the Harz ; and in the Echo district, Nevada. It is an ore of lead, but too rare to be of much importance. PYRARGYRITE Pyrargyrite. Dark ruby silver ; Sulphantimonide of silver, Ag 2 Sb 2 S 3 ; Ag = 59.9, Sb = 22.3, S = 17.8 ; Hexagonal ; Type, Ditrigonal Polar ; c = 0.7891 ; 0001 A lOTl = 42 20^, e A e' =_42 5', r_ A r' = 71_22', V A V' = 74 25'; Forms, aai20), mJlOlO), e (0112), r (1011), v_(2131) ; Twinning plane, 1120 also 1014 quite common, 1011 or 1012 rare ; Cleavage, r distinct, e less so ; Brittle, fracture conchoidal ; H. = 2.5 ; G. = 5.77-5.86 ; Color, grayish black to black ; Streak, purplish red ; Luster, adamantine ; Nearly opaque, deep red in thin splinters; - = .203; Optically (-). B.B. Fuses easily on coal to a globule and yields a white coat of antimony trioxide. In R. F. or with soda yields a malle- able button of silver. Soluble in nitric acid with the separation of sulphur and insoluble oxide of antimony. In the closed tube yields a red sublimate. General description. Crystals usually quite complicated both through twinning and the complexity of the forms. A large num- ber of forms have been described, but as formerly pyrargyrite and proustite were considered one and the same species, their individual forms have not as yet been entirely separated. Doubly terminated crystals are not common, but when they do occur they are generally supplementary twins with the basal plane as the composition face, in which case the two terminations are similar. The polarity of its symmetry is shown by the striations on the prism faces, which are not symmetrical to the basal plane. Also occurs compact, granular, or disseminated. Pyrargyrite occurs in veins associated with other silver minerals, as argentite, proustite, or native silver, with sulphides and arsen- ides ; calcite, barite, fluorite, or quartz are the gangue minerals. In such cases it has been produced by the interaction of antimonides on silver, in solution, either by precipitation or replacement at comparatively low temperatures, which also its synthesis in the laboratory would indicate. Often alters to argentite and forms pseudomorphs after native silver. SULPHO COMPOUNDS 323 Pyrargyrite is an important ore of silver occurring in most silver mines, as at Andreasberg and Freiberg, Saxony; at Pribram, Bohemia; Kongsberg, Norway; at various localities in Chili and Mexico. In the United States in the Ruby district, Colorado, associated with tetrahedrite ; in the Poorman's Lode, Idaho, in large masses ; also in Arizona and New Mexico. Artificially formed by precipitating a solution of silver with potassium sulph antimo- nate ; this precipitate is amorphous, but if mixed with sodium car- bonate and heated in a sealed tube above 80 the product becomes crystalline. PROUSTITE Proustite. Light ruby silver ; AgaAsSs ; Ag = 65.4, As = 15.2, S =^19.4; Hexagonal; Type, Ditrigonal Polar; c = .8039, 0001 A lOll = 42 51', e A e' = 42 46', v^v' = 74 _39 , r A rj = 72 12'; Forms, a (1120), e(0112), _r (1011), m (1010), (2131); Twinning plane, u (1014) and r (1011) common, e and c rare; Cleavage, r distinct; Brittle, fracture conchoidal; H. = 2-2.5; G. = 5.57-5.64 ; Color and streak, scarlet ; Transparent to trans- lucent, becoming black on exposure ; 3 being isomorphous with A1 2 3 . Clear corundum is variable in color and runs through the entire list of colored precious stones. When it is wished to convey the idea that the gem in question is corundum, the term "oriental" is prefixed, as "oriental topaz," " oriental amethyst," or " oriental emerald," etc. The true ruby is corundum; and when of that dark " pigeon blood " red, so much sought after and of the right transparency, it is the most expensive of gems, even surpassing the diamond of the first water in value. The ruby is colored with small quantities of chromium, while the sapphire and other colors are due to cobalt, titanium, or iron. Its physical properties, together with its wide range of colors, render the transparent varieties of corundum an ideal gem stone. It is the third hardest substance known, being surpassed only by the diamond and silicon carbide, a product of the electric furnace. Corundum is a primary mineral of such igneous rocks as granites, syenites, rhyolites, and rocks rich in alumina. In thin sections it appears transparent and nearly colorless, with or without crystalline outline, and there are no characteristic in- clusions. Relief is strong, and the interference colors are of the first order yellow or gray. In addition to being a primary mineral of igneous rocks, it is also a characteristic mineral of the belt of metamorphism, and is there associated with tourmaline, spinels, cyanites, garnets ; while in its decomposition by weathering it forms a whole series of aluminous minerals, as gibbsite, diaspore, margarite, muscovite, etc. Occurrence. Most of the gem material is of Eastern origin; the best rubies are found in the gravels of Irrawaddy River, near Mandalay, Burma, and in the crystallized limestone on its eastern bank ; the crystals are tabular in habit and associated with spinels and garnets. Those recovered from the river gravels are rounded and water-worn, but owing to their excessive hardness some still retain their crystalline faces. In Ceylon the gemstones are also re- covered mostly from the gravels of the Ratnapura and Rakwena districts. In the United States sapphires are found in the gravel bars of the upper Missouri River in Montana, and stones of gem value are mined in the Judith River valley, Montana. These are contained in a dyke cutting through a crystalline limestone. The dyke having weathered faster than the limestone may be traced by a depression across the country for five miles, and many sapphires have been taken from the piles of dirt at the entrance to the gopher OXIDES 343 holes. The decomposed dyke furnished much easier digging for these little animals than the hard crystalline limestone adjacent. Corundum is also found all along the Blue Ridge in Virginia, North and South Carolinas, and Georgia. Very fine blue specimens have been obtained at Sparta, New Jersey, and at various points in Sussex County. Most of the " adamantine spar " used in the United States is mined in Renfrew County, Ontario, where it occurs in a coarse pink syenite. Emery is a compact or granular variety of corundum which is mixed with a large proportion of oxides of iron. It is mined at Chester, Massachusetts, and at Peekskill, New York. In the latter locality it is associated with hercynite, magnetite, and garnet, and occurs as a segregation product from the norite of the Courtland series. Abroad emery is mined in Asia Minor, Tur- key, and Greece ; 60 per cent, of this product is exported to the United States. Corundum and emery are commercially used as an abrasive, espe- cially in emery paper for polishing and cleaning metals ; in wheels for sharpening steel tools, and in the cutting of glassware, though artificial corundum made from bauxite by heating it to a high tem- perature in an electric furnace is fast replacing the natural mineral. Artificial crystallized A1 2 O 3 may be produced by fusing equal parts of A1 2 03 and lead oxide, and allowing the fusion to cool slowly, when tabular crystals of corundum separate; if a little oxide of cobalt is added, they will be sapphire-blue ; if a little po- tassium dichromate is added they will be ruby-red. The so-called reconstructed rubies are formed in the oxyhydrogen flame; by rotating a small crystal rapidly in the flame with the temperature near the fusing point, it is then built up with fine particles of natural ruby, the color being regulated by the amount of chromium present. These artificial stones in their physical properties differ in no way from the natural ruby, and they puzzle even the expert to recognize them. They have however peculiar circular markings or pores, due to the rotation in the flame while in a semifused con- dition, which are a material aid in their identification. HEMATITE Hematite. Red oxide of iron, Fe 2 3 ; Fe = 70, = 30 ; Hexagonal; Type, Dihexagonal Alternating ; c = 1.3656; 0001 A 1011 = 57 37'; r A r' = 94; Common forms, c (0001), r(1011), e (01 II), u (10H) ; n (2243) ; m (10TO) ; Twinning plane r, both 344 MINERALOGY interpenetrating and polysynthetic ; Cleavage, none, parting caused by twinning; Brittle, fracture uneven; H. = 5.5-6.5; G. = 4.9-5.3, varies with the structure ; Luster, metallic, splendent in crystals, dull in massive form ; Color, dark iron-gray, red, reddish brown, to black; Streak, cherry-red; Opaque except in thin scales when blood-red by transmitted light ; = 2.94 ; 3, but there is no doubt but that ilmenite is a ferrous metatitanate and is out of place here as placed by Dana's classification. Magnesium and manganese are both isomorphous with ilmenite, and pyrophane is the manganous metatitanate, MnTi0 3 . Ilmenite occurs associated with magnetite and under the same conditions, as a primary constituent of igneous rocks ; as such it is one of the first minerals to separate from the magma. It is more abundant in the basic rocks, as the diorites/ diabases, and basalts. It also occurs in schists, gneisses, metamorphic rocks, argillites, and slates. In rock sections it is opaque and appears brownish by reflected light. When in crystalline outline it is elongated, but occurs more often as rounded grains and irregular masses, not to be distinguished from magnetite or chromite but by chemical tests. Ilmenite is often altered, resulting in a clear or translucent boundary, or area surrounding the opaque masses, composed of a highly doubly re- fracting substance termed leucoxene, formed by the decomposition of the ilmenite, and which has been identified as perovskite (CaTiO 3 ), as titanite, and again as anatase. Ilmenite occurs in large masses at Bay St. Paul, Quebec, also in Orange Co., New York, associated with serpentine, spinel, rutile, and chondrodite; at Litchfield, Connecticut; at Chester and South Royalston, Massachusetts. The largest crystals of ilmenite, some of which weigh sixteen pounds or more, have been found in a diorite at Kragero, Norway. As an accessory in igneous rocks ilmenite is very widely distributed. Ilmenite finds but little use in commerce; it is used as linings in puddling furnaces, but owing to the difficulty of handling it in the blast furnaces it is not used as an iron ore, though at the present time titanium steel is being tried for rails with encouraging results. Artificially ilmenite has been formed by heating a mixture of me- tallic iron, ferric oxide, and amorphous titanic oxide in a sealed tube to 270-300 C. OXIDES OF THE R0 2 TYPE CASSITERITE Cassiterite. Stream tin ; Tin binoxide, SnO 2 ; Sn = 78.6, O = 21.4; Tetragonal; Type, Ditetragonal Equatorial ; c = .6723; 001 A 101 =33? 54'; 110,111=46 27'; Common forms, 348 .MINERALOGY s (111), e (101), m (110) a (100). Twinning plane 101, both geniculate and cyclic; Cleavage, 110 imperfect; Brittle, fracture uneven; H. = 6-7; G. = 6.8-7.1; Luster, splendent adamantine ; Color, various shades of brown, red, and gray to almost black ; Streak, pale; n = 1.997. B.B. Infusible, reduced with soda and borax on coal yields malleable tin buttons. Insoluble in acids. General description. Crystals are short stout prisms with the prism faces striated parallel to the c axis, usually terminated with the two unit pyramids. Acicular crystals terminated by the pyra- mids (321) and (521) occur at Cornwall, England, also massive or FIG. 417. Cassiterite from Bohemia. The Upper Figures are Stream Tin from Mexico. granular. While cassiterite is found as a granular or disseminated primary accessory mineral in some igneous rocks, it is more often connected with the cavities and pegmatitic veins in the region of granitic masses which have been intruded in sedimentary forma- tions. Here its origin is the result of pneumatolytic agencies which have concentrated the tin on the border of the granitic mass, where it has been deposited in the veins, close at hand, of the dis- turbed area. In such veins it is associated with fluorite, tourma- OXIDES 349 line, topaz, and other rarer minerals, as wolframite, scheelite, or unraninite, which have been concentrated by the same agents. Owing to its high specific gravity and not being affected by weathering, cassiterite is left behind after most of the other minerals forming the rock mass have been decomposed and carried away ; it is thus mechanically concentrated in the bottom of streams as rolled, rounded, and water- worn pebbles (stream tin). It is from these alluvial deposits that a large amount of the tin of commerce is recovered. Little cassiterite is produced or mined in the United States ; small deposits are found in Lincoln County, North Carolina ; at Harney's Peak, South Dakota; near El Paso, Texas; and in the Seward Peninsula, Alaska. The world's supply is derived from the Malay Peninsula, Bolivia, Australia, and Cornwall, England. RUTILE Rutile. Dioxide of titanium, Ti0 2 ; Ti = 60, O = 40 ; Tetragonal ; Type, Ditetragonal Equatorial ; c = .644 ; 001 A 101 = 32 47' ; Common forms, s (111), e (101), m (110), a (100), Twinning as in cassiterite; Cleavage, 110 and 001 good; H. = FIG. 418. Rutile Crystals from Lynchburg, Virginia. 6-6.5 ;G. = 4.18-4.25; Brittle, fracture uneven; Color, shades of brown to nearly black ; Streak, pale brown or reddish ; Luster, adamantine, metallic in appearance; Translucent to opaque; = .287; Optically (+). 350 MINERALOGY B.B. Infusible. In the S. Ph. bead beside tin on coal yields a violet color when cold; the bead powdered and dissolved in concentrated HC1, then reduced with powdered tin, yields a violet solution. Insoluble in acids. General Description. Rutile occurs in all varieties of rocks, igneous, metamorphic, and sedimentary, either as short stout, or elongated and acicular prisms, with striations on the prism zone parallel to the c axis ; very often these long, hairlike crystals are found penetrating clear quartz, as at St. Gothard, Switz- erland, when the specimens are pol- ished and cut as ornaments. At this same locality small prismatic rutile crystals are found placed in parallel position on hex- agonal plates of hematite. At Tavetsch, Switzer- land, reticulated, platelike masses of elongated crystals, interlocking at the twinning angle of 65 35', occur and are known as sage- nite. Rutile in the United States has been mined in Vir- ginia, where it is ,,^^ found near Arring- ton in a pegmatite; at Nelson it is associated, in dykes, with apatite; at Lynchburg beautifully formed crystals with a steel- like luster, both simple and twinned, are common, as also in Alex- ander County, North Carolina, and at Graves's Mountain, Georgia. FIG. 419. Acicular Crystals of Rutile included in Quartz. Japan. OXIDES 351 As a secondary mineral rutile is derived from octahedrite and brookite, forming pseudomorphs after the latter, as at Magnet FIG. 420. Rutile Crystals from Graves's Mountain, Georgia. The Upper Twins are from the Tyrol. Cove, Arkansas. These three minerals are all dioxides of titanium, in different phases, and are quite often associated; rutile has the highest specific gravity and is the stable form at high tem- peratures. Octahedrite is also of tetragonal symmetry, while brookite is orthorhombic. Rutile may also be formed in the alteration of ilmenite and titanite, which process is reversed in the alteration of rutile. Chemically rutile gener- ally contains iron, to which the red or brown color is due ; the iron content is considerable in some cases and massive rutile may grade gradually into ilme- nite. Commercially rutile is the source of titanium salts, which are used as a yellow coloring agent in porcelain and to obtain the ivory- FIG. 421. Brookite from Arkansas. Magnet Cove, 352 MINERALOGY like color in artificial teeth. Titanium trichloride is replacing stannous chloride as a mordant in cloth printing. Ferro-titanium is used to produce a hard steel, with a high transverse and tensile strength; this more recent use may in the near future increase the demand for titanium minerals. PYROLUSITE Pyrolusite. Dioxide of manganese, MnO 2 ; Mn = 63.2, O = 36.8 ; Amorphous but often in pseudomorphs ; H. = 2-2.5, soils paper ; G. = 4.82 ; Color and streak, black ; Luster, metallic to dull, opaque. B.B. Infusible, shows manganese with the fluxes. In the closed tube yields little or no water. . Dissolves in HC1 with the liberation of chlorine. General Description. Massive, compact, fibrous, staiactitic, or dendritic. Oxides of manganese are usually associated with iron ores, having been formed in many cases by the interaction of the same agents. Pyrolusite is formed from hydrated oxides of man- ganese by loss of water. It has been thought that pyrolusite may be orthorhombic in symmetry, but it is probable that such crystals are pseudomorphs derived from manganite by the expulsion of its water. Oxides of manganese are concentrated at many localities either by replacement or by precipitation from solutions resulting from the decomposition of silicates or carbonates containing manganese. Such deposits are associated with limestone and sedimentary forma- tions, or form irregular pockets and nodules in clays. Such deposits extend all along the Appalachian and Piedmont regions, where they are mined commercially in the Blue Ridge Mountains of Virginia, and at Cartersville, Georgia. Pyrolusite is also mined at Bates- ville, Arkansas ; and Livermore, California. Cabinet specimens are obtained at Salisbury, Connecticut, and Stockbridge, Massachu- setts. QUARTZ Quartz. Silicon dioxide, Si0 2 ; Si = 46.7, O = 53.3; Hexag- onal ;_ Type, Trigonal Holoaxial; c = 1.100; Common forms, m (1010), r (1011), z (0111), x (5161) ,_ s (1121) ; 0111 A 0110 = 38 13'; 1010,5161 = 12 1'; 10TO A lI01 = 38 18' Cleavage, r perfect but difficult; Brittle, fracture conchoids! ; H. = 7; G. = OXIDES 353 2.65 ; Color, when pure, colorless or white, when impure, all shades ; Luster, vitreous, splendent to dull or greasy ; Streak, white, or in colored specimens very pale; Transparent to opaque; = 1.5532; (0 = 1.5441; -a>=.0091; Optically (+) ; Rotary polarization in thick sections. B.B. Infusible (1600) ; yields little or no water in the closed tube; when finely ground and fused with two volumes of soda on the platinum wire yields a clear bead when cold. Insoluble in acids except hydrofluoric. General description. Crystals are very common and well developed, usually elongated parallel to the c axis, combinations of the plus and minus rhombohedrons and the unit prism. The prism faces are often striated horizontally, which serves to identify the prism faces on distorted specimens. The trigonal pyramid s FIG. 422. Quartz Var. Rock Crystal. Hot Springs, Arkansas. and the trapezohedron x are common in certain localities, but are, like the large number of other forms that have been described, rare except for the few favored localities. When the two rhom- bohedrons are equally developed, the crystals have the appearance of being terminated by a hexagonal pyramid, but this is not pos- 2A 354 MINERALOGY sible in the type in which quartz crystallizes. When the rhom- bohedrons are unequally developed, the plus rhombohedron r is usually the larger and has a high luster, in fact is more perfect than the minus rhombohedron z, which may be dull, and faces occurring in the same zone with it and the prism face beneath it are also apt to be dull, while the faces appearing under r will be bright. Since the right and left trigonal trapezohedrons are rare forms or restricted to noted localities, it is not always possible to determine (6) (a) FIG. 423. Right- (a) and Left- (&) handed Smoky Quartz Crystals from St. Goth- ard, Switzerland. whether any given specimen is a right- or left-handed crystal; but when the trigonal trapezohedron is present, the crystal is a right- handed one when the face appears in the upper right-hand corner of the prism face, or just below the right-hand corner of r, and left-handed when in a similar position in the left-hand corner. The trigonal pyramid s is a member of the same zone as m, x, and z, and will appear between z and x; striations on s parallel to the intersection of s and r are characteristic, and will determine the character of the crystal when x is absent. Holding the crystal in the usual position, when these striae run from northeast to south- west, as directions are considered on a map, the crystal is right- OXIDES 355 handed ; if from northwest to southeast the crystal is left-handed. Again if the edges of the zone mxsz ascend to the right around the crystal, like a right-handed screw, the crystal is right-handed; if to the left, left-handed. Still a fourth method of distinguishing right and left crystals is by the corrosion figures which often appear on crystals as the result of some solvent, or are produced artificially by treatment with hydro- fluoric acid; these fig- ures are pointed at one end and broad at the other ; those on the right are minor images of those on the left. As the trapezohedral faces are a key to both the twinning of quartz and its right- or left- handedness, it should be remembered that four of these forms are possible. The positive forms occur the more often, and are situated below r, the positive right under the right-hand corner, and the positive left directly under the left-hand corner. The left-handed forms hold the same relation to the rhombohedral faces z. Twinning in quartz is very frequent, though it is not always to be recognized unless the trapezohedral faces are present. 1. Interpenetrating twins occur, where the twinning axis is normal to the prism edge. In such twins x will occur on adjacent prism faces, modifying the upper right-hand corner ; they may not appear on all six prism faces, but if they do appear on any two ad- jacent faces it is sufficient to establish the twinned nature of the crystal. In all such twins the rhombohedral faces are complex in nature, portions which are bright are r and portions which may be dull are z ; these areas are quite irregular and separated by curved and jagged boundaries, plainly shown in the photographs. Whether the twin consists of two right- or two left-handed individuals may be determined from the relation of x to the bright patches of the rhom- FIG. 424. Quartz Twinned, composed of Right- hand Individuals. St. Gothard, Switzerland. 356 MINERALOGY bohedral faces, as x will be under the bright areas in right- and under the dull areas in left-handed crystals. 2. Twins occur in which the face x modifies both upper or both lower corners of the prism face; these are twinned by reflection over a plane perpendicular to the prism face and parallel to the verti- cal axis. These are termed Bra- zilian twins and are often repeated, interpenetrating and quite irregu- lar, but the twins are always formed by the union of right and left individuals. In some cases the plane of reflection may pass through opposite prism edges, when four x faces will lie adjacent to a single and alternate prism edge. 3. Twins occur in which the twinning axis is perpendicular to the rhombohedral face 1122; after a revolution of 180 around this axis the vertical crystallographical axes of the two individuals will lie at 84 33'. This type of twins is usually flattened parallel to the prism face and they are known as the Japanese twins, as the most beautiful specimens are ob- tained from that country. Quartz is the most com- mon of all minerals. It is distributed universally and occurs under the most varied conditions. It is one of the essential minerals of granite, mica schist, and gneiss, while quartzite and sandstones may be almost pure quartz. In rock magmas Si0 2 takes FIG. 425. Quartz Crystals from near Rome, Italy. FIG. 426. Quartz twinned on 1122. Alaska. the part of an acid, and for this reason quartz is never found in the basic and dark- colored igneous rocks, as quartz is formed only in those cases where there is an excess of SiO 2 over the basic oxides. It is usually near the last to crystallize or separate in the solidification of a rock OXIDES 357 magma. It may therefore include within its mass all those min- erals which have separated previous to it, as the oxides of iron, rutile, apatite, zircon, mica, amphibole, and pyroxene; and in some instances it forms a ground mass in which the individual crystals of other minerals are imbedded. In the final solidification of acid igneous rocks, quartz usually preserves a balanced equilibrium with orthoclase, separating as a eutectic, which is well illustrated in the struc- ture of the micropeg- matites, where the crystals are so fine and intimately mixed as to be revealed only by the microscope. In thin sections quartz is colorless and transparent. Its index of refraction is so near that of Canada balsam that there is scarcely any relief, and the quartz grains when free of inclusions have the appearance of holes in the rock sections. In sections less than .04 mm. in thickness the interference is gray or yellow of the first order. It is often filled with small inclusions of liquids or gas. As a secondary mineral quartz may be formed by the solvent action of percolating waters containing carbon dioxide, decompos- ing complex silicates by carrying out the bases in solution, forming carbonates of the bases and quartz. The SiO 2 may dissolve also, to be later deposited in veins, cracks, and cavities, either in the form of quartz or as hydra ted silica (opal). Such quartz is asso- ciated with many ore deposits as the gangue mineral or vein filler ; it has a peculiar greasy luster and splintery fracture and is often termed vein quartz. The quartz grains of granites resist atmospheric weathering ; and when changes and decomposition of the other minerals are in prog- ress at the surface, these grains remain unchanged in the residue, forming the sands of the soil ; or when washed clean and redeposited FIG. 427. Enlarged Micropegmatite between Crossed Nicols. The Light Areas are Quartz, the Dark Orthoclase. 358 MINERALOGY by running water form the stratified sandstones in which silica deposited from solution may be the cementing agent. Varieties. Rock crystal is that clear, colorless, crystallized variety to which the sciences of mineralogy and crystallography owe so much. It has furnished convenient material to the scientist and physicist for experimentation and for apparatus since historic times, and it stands in its relation to crystallography as the frog does to the biological sciences. It has indeed furnished the name crystal, as the ancients believed it to be water which had been sub- jected to such a low temperature as to be no longer capable of re- turning to the liquid state. Nicolaus Steno in 1669 noted the similarity of the angles between crystal faces on quartz while cut- ting sections. Beautiful clear crystals of quartz occur in the calcareous sand- stone of Herkimer County, New York, known from their brightness as Herkimer County diamonds. These are in some cases chemi- cally pure SiO 2 , others are colored dark with carbonaceous inclu- sions, while others have cavities containing liquids in which bubbles may be rolled back and forth like the bubble of a spirit level. Evi- dently all these crystals have, from the character of the inclusions, been formed from solution and at a low temperature. This variety of quartz is in all cases considered to have been formed at a tempera- ture below that of 575 C., for when quartz is heated to a tempera- ture of 575 C., it passes over to another phase, /?-quartz, in which the physical properties are different from a-quartz, the phase stable below 575 C. Large crystals and thick sections of quartz in passing this inversion temperature are shattered, crack and fall in pieces; /3-quartz is hexagonal holoaxial, while a-quartz is trigonal holoaxial; and whenever the trigonal trapezohedral face x appears, that crystal must have been formed at a temperature below the inversion point and as a-quartz, since the trigonal trapezohedron is not a possible form in the hexagonal holoaxial type ; also the occurrence of two rhombohedrons r and z unequal in development and luster would indicate that the crystal had separated as a-quartz, since these faces in the hexagonal holoaxial type should be similar in all respects. Quartz of granite micropegmatites and some macropegmatites has been formed at a temperature above 575, as is shown by its frac- tured condition. Large clear crystals are found at the Hot Springs, Arkansas ; these have been formed in several stages of growth as is indicated by the internal crystalline outlines known as " phantoms," OXIDES 359 caused at this locality by fine crystals of chlorite being deposited on the crystal at different periods of its growth. More compli- cated crystals are found in North Carolina, showing rare forms. Quartz is quarried in Connecticut, Maryland, New York, Wis- consin, and North Carolina, for various purposes. When finely ground it is used as a filler in paints and scouring soaps. It is used in pottery and glass, and recently it has been fused and blown as glass, in chemical ware, such as evaporating dishes, flasks, cru- cibles, and ignition tubes, for the determination of carbon ; here it has the advantage, due to its low coefficient of expansion, of not being liable to crack when submitted to sudden changes of tem- perature ; even when at a bright red heat it may be plunged into cold water without the least danger of cracking. After fusion it has lost its crystalline structure and is amorphous silica, but on repeatedly heating and cooling, the molecules will rearrange them- selves and become crystalline, and then the tubes are liable to shatter on passing the inversion temperature of ft- to a-quartz. Colored quartz. Owing to small quantities of metallic oxides or organic matter as impurities quartz may appear in various colors. Some of the colored varieties have received special names, as citrine or yellow quartz, which is clear and transparent, in appearance very much like the topaz, and indeed when cut, pol- ished, and mounted in jewelry is sold in the trade as topaz, or false topaz. The best examples of this variety are obtained from Brazil. Smoky quartz is a dark colored variety of crystalline quartz which owes its color to organic matter or carbon compounds. The evenly colored transparent specimens are polished and valued as a semi- precious stone. Disentis and St. Gothard, Switzerland, are noted localities. These crystals generally have the faces x and s well developed. In the United States, it occurs at Pike's Peak, Colo- rado; in Richmond County, New York; and very large crystals have been obtained at Paradise River, Nova Scotia. Amethyst is a purple or bluish violet quartz which is colored with small amounts of manganese or possibly by organic matter. The color may vary greatly ; in most specimens it is unevenly distributed through the crystal, and is usually concentrated at the apex. Dark, evenly colored crystals are much prized as a semiprecious stone. The best colored specimens are from Siberia, India, Uruguay, and Brazil. Pale varieties are widely distributed. In the United States amethysts of good color are found in Lincoln and Macon counties, North Carolina; Nelson County, Virginia; Rabun 360 MINERALOGY County, Georgia; Thunder Bay, Lake Superior; also at Digby Neck, Nova Scotia. Other varieties are cryptocrystalline or aggregates of radiated, parallel, or matted fine fibers ; in all these the chemical properties FIG. 428. Onyx from Brazil. are the same as crystalline quartz, but they are softer and of a little lower specific gravity. Chalcedony is a translucent, concretionary form of Si0 2; usually light in color, often stalactitic or botryoidal, occurring as crusts lining cavities, and in geodes, which often contain large cavi- ties partly filled with water, which may be seen through the translucent wall on rolling the specimens back and forth. When very marked in color they have been given special names, as carnelian for the translucent red variety. Chrysoprase is light green, colored by the oxide of nickel. Bloodstone is chalcedony with small inclusions of red jasper. The banded varieties are agates, and when the bands are flat and OXIDES 361 regular it is onyx. At times the impurities are dendritic and ap- pear like pieces of moss inclosed within the specimen; these are moss agates. When dark in color and associated with limestones in nodules it forms flint. Lydian or touchstone is also a very dark, almost black, variety, used by the goldsmiths to test the purity of their gold alloys, by means of the color of the streak made by the metal in drawing it across the stone. Jasper is an opaque variety of impure SiO 2 of a red, brown ocher, gray, green, or black color. In addition there are pseudomorphs of SiO 2 after shells, wood (silicified wood), or bones ; and various minerals, as carbonates and sulphates, may be replaced by SiO2 from solution. TRIDYMITE Tridymite. Silicon dioxide, Si02 ; Hexagonal, Hexagonal above 130 C., below probably Orthorhombic ; 6 = 1.653; 0001 A 1011 = 62 21'; Common forms, c (0001), m (10lO) 2 a (1120), p(1011); Twinning plane 1016 and 3034 ; Cleavage, 1010 distinct ; ; Brittle, fracture conchoidal ; H. = 7; G. = 2.28-2.33 ; Colorless to white; Luster, vitreous ; Streak, white ; = 1.477. B.B. Like quartz, but soluble in boiling alkaline carbonates. Fuses at 1625 C. General Description. Crystals are small hexagonal tablets, combinations of the base, prism, and pyramid, or aggregations of these small scaly crystals. At ordinary temperatures tridymite is pseudo-hexagonal ; above 130 it becomes truly uniaxial, and below it shows low double refraction (.0018), is optically (+), and is prob- ably orthorhombic in symmetry. It is not a common mineral, but found in acid volcanic rocks and lavas, as in the lavas of Vesuvius and Krakatau, Obsidian Cliff, Yellowstone Park, and in some meteorites. It was discovered at Pachuca, Mexico, where it oc- curs in aggregates of twins ; in the cavities of an andesine rock. It has since been discovered in many localities connected especially with, and in cavities of, the more recent acid volcanic rocks. Tridymite and quartz are polymers of SiO 2 . The transition temperature between the two is near 800 C., quartz being the more stable phase below, and tridymite the more stable phase above that temperature. Artificially, tridymite has been formed both in solution and in dry fusion. When a silicate is dissolved in the salt 362 MINERALOGY of phosphorus bead, the silica left behind as the silica skeleton is in the form of tridymite. HYDRATED OXIDES BRUCITE Brucite. Hydroxide of magnesium, Mg(OH) 2 ; MgO = 69, H 2 O = 31 ; Hexagonal ; Type, Dihexagonal Alternating ; c = 1.208 ; r A r' = 97 38' ; 0001 A lOTl = 60 20' ; Common forms, c (0001), r (lOll), p (2021) ; Cleavage, basal perfect, laminae sectile ; H. = 2.5; G. = 2.4; Color, white, greenish or bluish; Streak, white; Luster, waxy to pearly; Translucent to opaque; = 1.579; - = .180; Optically (-). B.B. Infusible, whitens by loss of C0 2 and reacts alkaline with CARBONATES 387 turmeric paper. Insoluble in dilute HC1, but effervesces in hot acid. The concentrated HC1 solution yields a white precipitate with H 2 S0 4 (CaSO 4 ). FIG. 444. Dolomite. Traversella, Switzerland. General description. In habit, generally in simple rhombo- hedrons which may occur in complex aggregates, with curved, warped, or saddle-shaped surfaces ; this warped appearance of the crystal faces may occur in any of the species of the rhombohedral group of carbonates, but is es- pecially characteristic of dolo- mite and siderite. Dolomite, while placed in the calc te group, is not an isomorphous mixture of the two carbonates, but a double salt. This : s not only shown by the difference of sym- metry, but by the specific gravity of dolomite (2.85) being higher than would be required by the molecular mixture (2.843). That magnesium carbonates do enter the calcite molecule and form isometric mixtures is shown by f FIG. 445. Dolomite Var. Teruel, Spain. Teruelite. 388 MINERALOGY almost any analysis of calcite; most of the magnesian lime- stones are of this nature. Beds of dolomitic limestone are formed by the replacement of calcium carbonate with magnesium carbonate in coral reefs. This can take place in solutions of MgCl 2 and MgSO 4 common in sea water, and especially in the presence of C0 2 and at a temperature of 20 to 25 C. Mountain ranges in the West are formed by dolomite in which the rivers have eroded steep walled canons, which, with the cross valleys and tributary streams, cover the country with the peculiar butte formations ; such a section in the Tyrol is known as the Dolomites. An interesting variety of nearly black dolomite, teruelite, occurs at Teruel, Spain, in a matrix of gypsum. The crystals are combi- nations of an acute rhombohedron and the base equally developed, resembling very closely the octahedron, but the basal faces are always rough and quite different from the rhombohedron. Uses. Massive dolomites are very desirable as building stones. SIDERITE Siderite. Chalybite ; Spathic iron ore ; Ferrous carbonate, FeC0 3 ; FeO = 62.1 (Fe = 48.2), CO 2 = 37.9; Hexagonal; Type, Dihexagonal Alternating; c = .8184; 0001 A 1011 = 43 22' FIG. 446. Fluorite and Siderite. Harz Mountains, Germany. CARBONATES 389 51"; r A r' = 730'; Common forms, r(1011), c (0001) ; Twin- ning plane, e (0112) ; Cleavage, rhombohedral perfect; Brittle, fracture uneven ; H. = 3.5-4 ; G. = 3.83-3.88 ; Color, various shades of brown and gray ; Streak, white unless oxidized, when it may be brown; Luster, vitreous; Translucent; o = 1.873, c ;= 1.633; <>- = .240; Optically (-). B.B. Loses C0 2 and blackens. In R. F. becomes magnetic. Fuses at 4.5 and reacts for iron with the fluxes. Effervesces in dilute HC1, especially when hot, and dissolves completely when pure. General description. Crystals are simple rhombohedrons or complex crystals, which are curved rhombohedrons as in dolomite. Simple rhombohedral crystals of half a pound in weight occur embedded in the cryolite of Ivigtut, Greenland. At the Buckler's FIG. 447. Cryolite and Siderite from Ivigtut, Greenland. Mine, Cornwall, England, crystals, combinations of the rhombo- hedron, base, hexagonal prism, and a scalenohedron occur, but forms other than the rhombohedron are rare. Siderite is also massive, granular, or disseminated. The various colors depend upon the amount of oxide of iron, or oxidation, and at times it is very dark, due to the presence of hema- tite or limonite, to which it is easily transformed by oxidation. 390 MINERALOGY \Yhen mixed and crystallized with MgCO 3 , it forms mesitite, as at Traversella in Piedmont, which has a rhombohedral habit and is theoretically iron and magnesium carbonates in molecular propor- tions. Ankerite is a mixture of calcium, iron, and magnesium car- bonates, occurring with iron ores and siderite, as in Styria, Siegen, Nova Scotia, and northern New York. Siderite like other carbonates is deposited from the bicarbonate solution, but in this case it must be laid down under non-oxidizing conditions or in the presence of organic matter, otherwise hematite or limonite will form, as ferric carbonate is not known as a dry salt. It is also formed by the replacement of calcium in calcite FIG. 448. Siderite, showing the Hexagonal Prism and Base. Cornwall, England. and limestones and is therefore an important constituent of many sedimentary rocks, especially those of the coal formations. Siderite as a vein filler is associated with many ore deposits. Uses. It is mined as an iron ore in Cornwall, England, but it is of little importance as an iron ore in the United States. For- merly it was mined at Roxbury, Connecticut. Artificial. Microscopic crystals may be obtained by precipitat- ing a solution of ferrous sulphate with sodium bicarbonate and heating in a sealed tube to 150 for several hours. RHODOCHROSITK Rhodochrosite. Manganese carbonate, MnCO 3 ; Mn = 61.7, C0 2 = 38.3 ; Hexagonal ; Type, Dihexagonal Alternating ; c = 8184; 0001,1011=43 22' 50"; r A r' = 73 0'; Forms, CARBONATES 391 r (lOll), e (1112); Cleavage, rhombohedral perfect; Brittle, fracture uneven ; H. = 3.5-4.5 ; G. = 3.45-3.60 ; Color, various shades of red or pink, yellowish gray, brown when oxidized ; Streak, white; Transparent to opaque; = 1.597; Optically ( ). B.B. Blackens and decrepitates, infusible ; with the fluxes reacts for manganese. Dissolves in hot dilute HC1 with effer- vescence, yielding carbon dioxide. General description. Crystals are not common, but in habit are simple rhombohedrons ; other forms are rare. Complex warped rhombohedrons of a beautiful pink color occur in Saguache County, also dark rose-colored transparent rhombohedrons asso- ciated with cubes of pyrite at Alicante, Lake County, Colorado. At Butte, Montana, the simple rhombohedrons are coated with quartz. Fine specimens are obtained from Franklin Furnace, New Jersey, where large masses of cleavable calcite occur, colored pink with manganese carbonate which has crystallized with it as an isomorphous mixture. These mixtures of calcium and manganese carbonates are given definite names when the amount of manganese reaches a consider- able percentage, as manganocalcite for the 25 per cent, mixture, or manganosiderite when much iron is present. Rhodochrosite is formed under the same conditions as the other rhombohedral carbonates, as it is more stable than the ferrous car- bonate; when they are contained in the same solution, the iron is deposited first, and the manganese may be carried in solution much farther from the original source. Owing to this difference of stability in solution the two are almost always deposited apart. Artificial. Microscopic rhombohedrons may be formed by heating a solution of manganese chloride and sodium carbonate in 'a closed tube to 160 C. SMITHSONITE Smithsonite. Dry bone, Carbonate of zinc, ZnCOs ; ZnQ = 64.8, CO = 35.2; Hexagonal; Type, Dihexagonal Alternating; c = .8063; 0001 A 1011 =_4257'20"; r A r' = 72 20'; Forms, r (1011), e (0112), v (2131); Cleavage, rhombohedral perfect; Brittle, fracture, uneven ; H. = 5 ; G. = 4.3-4.45 ; Color, white, yellow, green, brown ; Streak, white or pale ; Luster, vitreous to dull; Transparent to opaque; = 1.618; Optically ( ). 392 MINERALOGY B.B. Infusible; reduced with soda and borax on coal yields a white coat which becomes green with cobalt solution (Zn). Dis- solves in hot dilute HC1, completely when pure, with effervescence yielding carbon dioxide. Highly colored green varieties contain copper ; the brown, iron or manganese. General description. Crystals are not common, but occur in small simple rhombohedrons, or scalenohedrons, as at Friedensville, Pennsylvania. Smithsonite is more often massive, incrusted, botryoidal, banded, or earthy, than crystalline. The pure zinc carbonate is white, and the mineral owes its various colors to the impurities and isomorphous carbonates which crystallize with it. A very beautiful green variety containing considerable copper occurs at Kelley, New Mexico ; also in Greece. The bright yellow variety, known in Arkansas and southern Missouri as " turkey-fat ore/' is colored with the sulphide of cadmium, greenockite, which is associated in small amounts with the zinc ores of this region. Smithsonite is connected with most zinc ore deposits as a second- ary mineral, a surface oxidation product derived from sphalerite, after which pseudomorphs are common. Zinc may be carried, by percolating waters, in solution, either as the sulphate or bicarbonate, to be precipitated as sulphide or by replacement of carbonate of lime as carbonate. Zinc ores therefore fill veins, pockets, or lenses in limestones, and may have been concentrated from the lime- stone itself or carried to it from neighboring regions. As a vein mineral Smithsonite is associated with calcite, barite, siderite, galena, pyrite, most of which have originated through the same agencies. Smithsonite as a zinc ore is of minor importance in the United States. It is mined in Virginia, Pennsylvania, Arkansas, and Colo- rado. Its synthesis is effected by the same means as the other car- bonates of the group. Sphserocobaltite, a carbonate of cobalt isomorphous with the calcite group, is found in small rhombohedral crystals at Schnee- berg in Saxony. Some rhodochrosites contain small quantities of cobalt. ARAGONITE Aragonite. Calcium carbonate, CaC0 2 ; CaO = 56 L C0 2 = 44 ; Orthorhombic ; Type, Didigonal Equatorial ; & : b : c = CARBONATES 393 .6224 : 1 : .7205 ; 100 A 110 = 31 54' ; 001 A 101 = 49 10' 42"; 001 A Oil = 35 46' 30"; Common forms, c(011), b (010), m (110), o (112), other forms numerous but not common ; Twinning plane, 110, often repeated; Brittle, fracture subcon- choidal;H. =3.54; G. = 2. 93 L 2.95; Color, white, yellow, gray, sometimes green; Streak, white; Luster, vitreous ; Trans- parent to opaque; a = 1.529; p = 1.681; y = 1.685; y - a = .156; Optically (_) ; 2V = 18 11'; Axial plane parallel to 100; Bx a parallel to c. B.B. Like calcite. General description. Crystalline habit prismatic, combinations of the unit prism with the brachy- pinacoid; as the prism angle is 116 12', the crystals have a hexagonal outline. Such combinations terminated by the base are common at Herrengrund, Hungary. At Cleator, England, crystals occur illustrating the chisel-shaped habit, where the ter- mination is formed by the long steep pyramid (991) and dome (091), in which the dome predominates, giving the crystal the flattened or chisel appearance. Twinning in aragonite adds to the hexagonal appearance, as the composition plane is parallel to the unit prism; when repeated and joined in parallel position, yield short stout hexagonal prisms terminated by a flat face, the base. The complexity of such specimens is easily seen by the striations on the base, or by the concave or offset prism faces. Aragonite, as the unstable form of calcium carbonate at ordinary temperatures, is less common than calcite. It is formed from hot solutions, around thermal springs, in crusts, botryoidal shapes, and peculiar coral-like masses known as "flos-ferri," orflowers of iron,from their form and association with beds of iron ore . The ' ' sprudelstein' ' of the hot springs of Carlsbad, Bohemia, is aragonite, as well as the calcic skeleton of corals and the pearly lining of many shells. FIG. 449. Aragonite from Herrengrund, Hun- gary. 394 MINERALOGY Heretofore all non-crystalline deposits of calcium carbonate were termed aragonite, but calcite is deposited in a massive state a t l w temperatures also, and the iden- tity of such forma- tions should be determined by ac- tual tests (see calcite) . Aragonite passes over to cal- cite, with which it is dimorphous, by a change of its physi- cal properties, form- ing pseudomorphs, the chemical com- position remaining unchanged. Good aragonite crystals are not common; those of Herrengrund are probably the best. Pseudohexagonal forms occur at Aragon, Spain, where the mineral was first discovered. At Girgenti, Sicily, it occurs associated with celestite, sulphur, and gypsum. Often associated with zeolites, and in clays with gypsum. Artificial. When a solution of calcium bicarbonate is heated above 30 C., aragonite separates; below 30, calcite separates. FIG. 450. Chisel-shaped Crystals of Aragonite from Cumberland, England. WITHERITE Witherite. Barium carbonate, BaC0 3 ; Ba = 77.7, C0 2 = 22.3 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = .6032: 1 : .7302; 100 A 110 = 31 6'; 001 A 101 = 50 26' 30"; 110 A 001 = 36 8' 15"; Common forms, c (001), m(110), b (010) ; Twinning plane parallel to 110; Cleavage, b distinct, m imperfect ; Brittle ; Fracture, uneven ; H. = 3-3.75 ; G. = 4.29- 4.35 ; Luster, vitreous ; Color, white, gray, or yellow ; Streak, white ; CARBONATES 395 Transparent to translucent; P = 1.740; Optically ( ); Axial plane parallel to 010; Bx a = c; 2E = 26 30'. B.B. Fuses easily to a globule and yields a green flame in the forceps. After ignition reacts alkaline with turmeric paper. Effer- vesces with cold dilute HC1; the solution greatly diluted yields a white precipitate with H 2 SO 4 . General description. Witherite occurs, as a rule, not in well- formed crystals, but in masses with a radiated, columnar, botryoi- dal, or granular structure. When crystals are well developed, they are pseudohexagonal in shape, from the repeated twinning parallel to 110. Its genesis and associations are like aragonite, but it occurs more often in veins with barite and galena. By the action of waters containing sulphates in solution witherite will be transformed to barite. Witherite is not a common mineral, and it occurs in the United States in but few localities, as at Lexington, Kentucky, and near Thunder Bay, Lake Superior. At Fallowfield, England, it is mined commercially; splendid crystals are otained at this locality. It is isomorphous with aragonite and alstonite or bromlite; (Ba, Ca) CO 3 is such a mixture, found in pseudohexagonal crystals at Alston Moore, England, while barytocalcite, BaCa (CO 3 ) 2 , is probably a double salt and monoclinic in symmetry. Witherite is the source of barium salts ; it is used as a substitute in paints, and in powder as a rat poison. Artificially it is formed when the carbonate is fused with sodium chloride, or in the precipitation of a hot solution of barium salts by ammonium carbonate. STRONTIANITE Strontianite. Strontium carbonate, SrC0 3 ; Sr = 70.1, C0 2 = 29.9 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = .6090 : 1 : .7238 ; 100 A 110 = 31 20' 30" ; 001 A 101 = 49 55' 30"; 001 A Oil = 35 54'; Common forms, c (001), b (010), m (110), i (021) ; Twinning plane, 110; Cleavage, prismatic good; Brittle; Fracture, uneven; H. =3.5-4; G. = 3.68-3.71 ; Luster, vitreous; Color, white, yellow, gray, pale green; Streak, white; Transparent to translucent; a= 1.515; p = 1.664; -y = 1.666; 396 MINERALOGY Optically ( ); Axial plane parallel to 100; Bx a = c; 2E = 10 36'. B.B. Fuses in the forceps with difficulty and yields a deep red flame ; after ignition reacts alkaline with turmeric paper. Effer- vesces in cold dilute HC1. General description. Crystals are often sharply pointed in habit, caused by the development of the acute pyramid and brachy- dome, as at Hamm in Westphalia, also columnar, fibrous, or granular. It is isomorphous with barium and calcium carbonates and usually contains some of these salts. In its occurrence it is asso- ciated with galena in ore deposits, and with barite and celestite ; from the latter it may have been derived as a secondary product, as at Mount Bonnell near Austin, Texas; occurs also in Jefferson County, New York, and in Monroe County, Michigan. Strontianite is the commercial source of the salts of strontium, which are used to produce the red fires in pyrotechnics. The hy- drate is used in the beet sugar industry to precipitate the sugar from the molasses ; for this purpose barium hydrate, owing to its cheapness, is sometimes used as a substitute. Artificially produced like witherite. CERUSSITE Cerussite. Carbonate of lead, PbC0 3 ; PbO = 83.5, C0 2 = 16.5 ; Orthorhombic ; Type, Didigonal Equatorial ; fi : b : c = .6099 : 1 : .7230 ; 100 A 110 = 31 22' 55" ; 001 A 101 = 49 51'; 001 A 011=35 52'; Common forms, c (001), m(110), b (010), P (111) i (021) ; Twinning plane, 110; Cleavage, m and i distinct ; Brittle ; Fracture, conchoidal ; H. = 3-3.5 ; G. = 6.46- 6.57; Color, white, yellow, or gray; Streak, white; Luster, ada- mantine to resinous ; Transparent to opaque; a = 1.804; p = 2.076; v = 2.078; Optically (-); Axial plane parallel to 010; Bxa = c; 2V = 8 14'; 2E = 17 8'. B.B. Darkens in the closed tube and decrepitates. On coal in R. F. reduces to malleable lead and yields a lead coat. Dis- solves in dilute nitric acid with effervescence, yielding carbon dioxide. CARBONATES 397 General description. Crystals are tabular, pyramidal, or elongated prismatic in habit. When pyramidal in habit, like other members of the group, they are pseudo-hexagonal in appearance, from the equal development of the pyramid ,and brachydome, as at Mies in Bohemia. When tabular, it is very apt to form stellate groups by repeated twinning after the aragonite method, but here the space between in- dividuals is not completely filled as in aragonite, as at Mies and Pribram, Bo- hemia. When elongated in habit, the prism zone is deeply striated lengthwise, FIQ 45L _ Cerussite , Twing MieSi Bohemia as at the old Wheatley Mine, Chester County, and Phoenixville, Pennsylvania. It occurs also in crusts, stalactitic, granular, or massive, often colored green or blue with copper carbonates. Cerussite is associated as a secondary product in the superficial areas of lead deposits where it is formed from galena by oxidation and the action of carbonated waters, which may also form carbon- ates from anglesite, the sulphate of lead, after which many pseudo- morphs are found. Next to galena cerussite is the most important ore of lead and is mined at Leadville, Colorado; Arizona; and Utah. Artificially it may be formed as aragonite. MALACHITE Malachite. Basic carbonate of copper, (Cu . OH) 2 CO 3 ; CuO = 71.9, _C0 2 = 19.9, H 2 = 8.2; Monoclinic; Type, Equato- rial; a : b : c = .881 : 1 : .401 ; p = 61 50' = 001 A 100; 100 A 110 = 37 50'; 001 A Oil = 19 29'; Common forms, c(001), a (100), b (010), m (110); Twinning plane, 100; Cleavage, basal perfect, b less so; Brittle; Fracture, uneven; H. =3.5-4; G. = 3.9-4; Color, bright green; Streak, pale green; Luster, vitreous, silky 398 MINERALOGY to dull and earthy; Translucent to opaque; = 1.87; Optically (-) ; Axial plane parallel to 010; Bx aA c = 23 39' in front; 2E = 89 18'; 2V = 44 7'. B.B. Blackens and fuses. In R. F. on coal yields copper and a green flame. In the closed tube yields water, dissolves in dilute HC1 with effer- vescence, yielding car- j bon dioxide. *n General description. o Crystals are seldom '<% distinct individuals, but grouped in tufts, divergent or elongated, ri acicular, and radiated, ^ as at Betzdorf, in 2 Westphalia; also bot- ryoidal, stalactitic, nodular, and curvi- linear masses, formed with concentric layers of different shades of green, are common, as at Bisbee, Arizona. Malachite is a second- ary mineral formed as an oxidation prod- uct of other copper ores, as cuprite, chalcocite, chalcopyrite, or melaconite, by the action of percolating waters charged with carbon dioxide, and is characteristic of the surface workings of all copper deposits. Numerous pseudomorphs of malachite, especially after cuprite, occur, as at Chessy near Lyons, France, where there are beautiful examples of octahedrons and rhombic dodecahedrons, some of which are only coated with a crust of carbonate, while the interior is still unaltered cuprite. Octahedral pseudomorphs are also found at Bisbee, Arizona. Malachite is of common occurrence in many copper localities ; CARBONATES 399 the most beautiful specimens in the United States are obtained at Bisbee, Arizona, The massive banded variety occurs in the Urals in Russia, where it is much prized as an ornamental stone, used in the veneering of vases, table tops, and decorations in build- ings. The large in- terior columns of St. Isaac's Cathedral at St. Petersburg are of malachite. Malachite is also a valuable ore of copper. Artificial malachite is formed upon heat- ing a solution of cop- per bicarbonate. FIG. 453. Malachite with Concentric Bands of Various Shades of Green. Bisbee, Arizona. AZURITE Azurite. Chessylite ; Basic copper carbonate, Cu(OH) 2 . 2 (Cu- C0 3 ) ; CuO = 69.2, C0 2 = 25.6, H 2 O = 5.2 ; Monoclinic ; Type, Equatorial ; a : b : c = .850 : 1 : 1.880 ; P = 87 36' = 001 A 100; 100 A 110 = 40 21'; 001 A 101 =44 46'; 001 A Oil = 41 21'; Common forms, c (001), a (100), m(110), p (021) ; Twinning plane v (201) ; Cleavage, p perfect, a less so ; Brittle ; Fracture, conchoidal ; H. = 3.5-4 ; G. = 3.77-3.83 ; Color, azure- blue to deep blue ; Streak, smalt-blue ; Luster, vitreous to dull ; Translucent to opaque ; Optically (+) ; Axial plane perpendicular to 010; Bx aA c = - 12 36'; 2E = 151. B.B. Like malachite. General description. Well-developed crystals are more com- mon than those of malachite. They are varied in habit, and highly modified, often tabular combinations of the base and unit prism with a pyramid and dome, with the base striated, as at Bisbee, Arizona. Peculiar complex aggregates, two inches in diameter, some rhombohedral, others rounded, occur in the claylike pockets in the Copper Queen mine, Bisbee, Arizona ; also at Chessy, France. 400 MINERALOGY Azurite is also radiated, massive, stalactitic, granular, earthy, or botryoidal, with banded concentric layers like malachite, from which such specimens may form by a loss of CO 2 and hydration. FIG. 454. Azurite Crystals. Bisbee, Arizona. Azurite is a secondary mineral formed from other copper minerals in which the chemical reactions have gone a step farther than in malachite, and some of the C02 has been replaced by hydroxyl. Azurite is widely distributed in the superficial workings of almost all copper deposits. Commercially it is an important ore of copper, and, like malachite, the well-banded and colored specimens are used in the manufacture of trinkets, ornaments, and the less expensive jewelry. NATRON sodium carbonate, Na^COa . 10 H 2 O ; 15.4, H 2 O = 62.9 ; Monoclinic ; Type, c = 1.482 : 1 : 1.400; P = 58 52' = 51 46'; 001 A Oil =50 10' ; Common forms, b (010), m (110), e (Oil) ; Cleavage, basal distinct ; Brittle ; Fracture, conchoidal; H. = 1-1.5; G. = 1.42-1.46; Color, white Natron. Hydrous Na^O = 21.7, CO 2 = Equatorial ; a : S : 001 A 100; 100 A 110 CARBONATES 401 to gray or yellow ; Streak, white ; Luster, vitreous to dull ; Trans- parent to earthy. B.B. Soluble in water, yielding an alkaline solution ; effer- vesces with dilute acids. In the forceps a sodium flame. A strong alkaline taste. TRONA Trona. Hydrous sodium bicarbonate, Na 3 H(C0 3 ) 2 . 2 H 2 ; Na 2 O = 41.2, CO 2 = 38.9, H 2 O = 19.9; Monoclinic; Type, Equatorial ; a : b : c = 2.846 : 1 : 2.969 ; p = 77 23' = 001 A 100 ; Common forms, c (001), a (100), o (111) ; Cleavage, a perfect; H. = 2.5-3; G. = 2.11-2.14; Color, white, gray, yellowish ; Streak, white; Luster, vitreous; Translucent. B.B. Like natron. GAYLUSSITE Gaylussite. Hydrous sodium calcium carbonate, Na 2 Ca- (CO 3 ) 2 . 5H 2 ; CaC0 3 = 33.8, Na 2 C0 3 = 35.8, H 2 O = 30.4 ; Monoclinic ; Type, Equatorial ; a : b : c = 1.489 : 1 : JL.444 ; P = 78 27' = 001 A 100; 100 A 110 = 55 35'; 001 A 101 = 49 44' ; 001 A Oil = 54 45' ; Common forms, c (001), e (Oil), m(110), r(112); Cleavage, prismatic perfect, basal difficult; Brittle; Fracture, conchoidal; H.= 2-3; G. = 1.93-1.95; Color, white, gray, yellowish; Streak, white; Luster, vitreous; Trans- lucent. B.B. Whitens and fuses to a white enamel, yielding an in- tense yellow flame (Na) ; the residue after fusion reacts alkaline with turmeric paper. Soluble in acids with effervescence; the solution made alkaline with ammonia yields a precipitate with ammonium oxalate (Ca) . General description. Natural soda occurs in numerous localities where the dryness of the climate has permitted the con- centration or complete evaporation of large bodies of saline waters. In the West such deposits occur in Wyoming, Nevada, Utah, and at Mona, Borax, and Owens lakes, California. Most of these localities have been worked commercially for the sodium carbon- ates they contain. It has been estimated by Loew that Owens 2D 402 MINERALOGY Lake contains in solution 22 million tons of dry sodium carbonate. Near Ragtown, Nevada, crusts of trona exists over the surface of Soda Lake, of sufficient thickness and strength to bear the weight of a man. A third sodium carbonate, thermonatrite, Na^COs . H 2 O, with one molecule of water, is deposited from these solutions under fa- vorable conditions. Of the three carbonates, trona is the first to be deposited ; as concentration increases, the very soluble natron separates, mixed with sulphates and chlorides. Gaylussite is deposited in these lakes near the entrance of small streams or springs ; as the waters of these, carrying calcium in solution, are mixed with the strong soda solution, the calcium salt is deposited as gaylussite. All these large bodies of alkaline carbonates have been attributed to the concentration of solutions from volcanic rocks, or by the interaction of calcium bi carbonates in solution with alkali sulphates and chlorides. CHAPTER X SILICATES, TITANATES, ETC. FELDSPARS AND LEUCITE THE FELDSPARS THE feldspars constitute a most important group of rock-form- ing minerals, so important that not only is nearly 60 per cent, of the igneous rocks composed of feldspars, but their classifica- tion depends to a great extent upon the quantity and species of feldspar that is present. They are isomorphous in the fullest sense of the term, and even though they belong to both the mono- clinic and triclinic systems and are also salts of two silicic acids, yet they mix in all proportions, forming solid solutions. One species grades gradually into the others. The various mixtures which seem to be more constant in nature have been given special names, and in the past were considered as distinct species. From a chemical standpoint there are four species, two of which, orthoclase and albite, are salts of the trisilicic acid (H 4 Si 3 O 8 ) while the other two, anorthite and celsian, are salts of orthosilicic acid (H 4 Si0 4 ). Since they form a compact isomorphous group, they are very similar in all their physical and crystallographical properties. The following table will show very clearly these relations and will also serve for their microscopic indentification. ORTHOCLASE Orthoclase. Potassium aluminium trisilicate ; KAlSisOg ; K 2 O = 16.9, A1 2 O 3 =18.4, SiO 2 =64.7; Monoclinic; Type, Digonal Equatorial ; a : b : c = .658 j_l : .555 ; p = 116 3' = 001 A 100 ; 100 A 110 = 30 36' ; 001 A 101 = 50 17' ; _001 A Oil = 26 31' ; Common forms, c(001), b(010), m(110), x(101), y (201), n(011), o (HI) ; Twinning common, composition face b twinning axis c (Carlsbad twins), composition face n with twinning axis n 403 010 v TOO i ^ s f r & ll^^s 05 1 i & f ? a c, ^^^ss 05 o 3 J co 2 ^ S5 ^ ^? ^" $ 1-1 1-1 rH T-l ""* ^H *"* R ^ & ? r s ^ 05 g g ^ J O i Illils 2H " K * " * 1 1 i 1 1 1 1 ivoiido i i + i + i + d s ; as ^ < 3S fl O PL) CQ Id |8 HH o 2-i 00 t^ 00 % & 55 1 B 1 q q q q i a ! (N CO 00 O ' ca iO 11 ! 5 1 2 11 ! Z O!S% CO r^ t>; o co 01 q AXIAVHO 1 1C 05 1C CD CO t- t^ CO II WSIVX 6Q 5^Qc5 if ?$$< 314.411 ] about 1150 llo |o, IJ 2 |" a b ^ 5 k 1 1 1 + 2 V 1" j o o o o o o o o o coo o o" o" o" s 5 o ooo 2.2,2- H H H o. O 1> t~- INDICES OF R-EFRACTION FOR SODIUM LIGHT .+ 1 + ' + + + + \- + ii e- + 1 1 I + 1 q q q q (N CD 01^000 CO IN CO Ol M -r f- (>. t^ l>- X i 1 i s 1 5 cq coco t>t>-r>. Sccco cc q COCO H 1 oc co II II II II II II II II II II II II II CO. ?~ 00. co CO O 1-H Tf oo M* SB 1 CO IO lOCO cox II II f-^COcC'-H'-^C:C i Tf:y"' v i oooecoSSwoSaS H II II II II II II II II II 2 D (N W CO I 1 i 1 ll 11 8 s CO IN O d CO > * "fl "5 | fl - o -0 S J> i oo I s - I -S i 00 oo H* oo o 1 22 J 10 H 1 L PQ o ^ Sj "^ T-4 1 1 H Q | CO t> So j o o O o T:' o %& o o O o o i fl o E CO 1 00 00 1 I s i very large i HaxovavHO + 1 * 1 i -H ' ? + wssss ll "5 1 I i q q 1 jf 2 1 111 111 11 11 ro co -* >o <-! 00 CJ = 1.584; = 1.578; = 1.5416; = 1.5376; o>- = .004; Optically (-). B.B. Fuses quietly at 3.5 (1100), yielding a yellow flame (sodium). Gelatinizes with HC1; the solution freed of silica yields a precipitate of aluminium hydroxide with ammonia. General description. Crystals when developed are short, stout, hexagonal prisms terminated by the base or tabular parallel to the base. Several pyramids of both the first and second orders have been described on small crystals from the lavas of Monte Somma, Vesuvius. Elseolite is the more common granular or massive form of nepheline, of the older syenites, in which it occurs at times as the dominant mineral. Chemically nepheline is probably a mixture of sodium aluminium orthosilicate (NaAlSi0 4 ), which has been produced artificially, FIG. 482. Nepheline in a Section of Phonolite. The Small Square Crystals are also Nepheline. The Elongated Crystals are Feldspar. but is not known as a mineral, and potassium aluminium orthosili- cate (KAlSi0 4 ), which is the mineral kaliophilite, with possibly a little excess SiO 2 in its molecule. Eucryptite is the lithium salt ^), while cancrinite, H 6 NaeCa(NaC03) 2 Al 8 (Si04)9, is a min- 442 MINERALOGY eral very similar in physical and crystalline properties, as well as associations, but contains carbonates in its composition. In sections, nepheline when crystalline is either square or hex- agonal in outline, colorless, with no relief ; cleavage is not marked except when decomposition has begun. Inclusions are not charac- teristic except that in some occurrences it may be filled with dust- like inclusions of glass and gas bubbles, in concentric bands or zonal. Interference color very low grays of the first order, and extinction parallel to the cleavage or symmetrical. Nepheline is a primary mineral of many igneous rocks, especially those rich in alkalies and low in silica. It seems to form in those magmas in which the sodium is in excess of that required to pro- duce feldspars, separating from the magma in most cases just before the feldspars and directly after the sodalite group, with which it is frequently associated, as in the lavas of Vesuvius. At Litchfield, Connecticut, it is associated with cancrinite in an elseo- lite syenite. In syenites the massive elseolite is characteristic, lending its name to the group, elseolite syenites. In weathering, zeolites, especially natrolite, result ; but the forma- tion of sodalite by the addition of chlorine may be the first step in this reaction. Artificially nephelite has been produced by a fusion of its con- stituent oxides at a low temperature, from which crystals easily separate. GARNETS Garnets are orthosilicates of the general formula R // 3 R /// 2 (Si0 4 )3, in which R" may be Ca, Mg, Fe", Mn, and R'" = Al, Fe'", Cr ; Isometric; Type, Ditesseral Central; Common forms, d(110), n(211), s(321); Twins rare; Cleavage, dodecahedral sometimes distinct; Brittle; Fracture, conchoidal ; H. = 6.5-7; G. = 3.15- 4.3; Color, all colors; Streak, white or pale; Transparent to opaqus; Luster, vitreous to resinous; n = 1.7-1.8. B-B. Fuses at three, except uvarovite which fuses at six. After fusion gelatinizes with HC1, otherwise not much affected by acids. Those containing much iron become magnetic after fusion in 0. F. General description. Crystals are common and in some instances very large, up to a foot in diameter; in habit either rhombic dodecahedrons or tetragonal trisoctahedrons or combina- SILICATES, TITANATES, ETC. 443 tions of these two forms. Other forms are rare ; the cube occurs on crystals from Mill Rock, New Haven, Conn., and the octahedron on crystals from the isle of Elba. Striations appear on the dodecahedral face parallel to the long diagonal, and on the trisoctahedral face parallel to its intersection " * FIG. 483. Garnet Crystals in Schist. Southbury, Connecticut. with the rhombic dodecahedrons ; these striations appear on the weathered or water-worn crystals also. The garnets form a very compact isomorphous group in which there is a gradual transition from one variety to the other. They are as a rule grouped under six heads, with varieties under each of these. I. Grossularite, Calcium aluminium garnet, Ca 3 Al 2 (Si0 4 )3 ; CaO = 37.30, A1 2 3 = 22.69, Si0 2 = 40.01; H. = 6.5; G. = 3.5; Color, white, amber yellow to cinnamon brown, sometimes green when chromium is present. Cinnamon stone, or hessonite, is a brown variety, beautiful specimens of which are obtained from Ala, Piedmont ; also Ceylon ; and Bethel and Rumford, Maine ; Amity, New York, and many other localities, as it is a common variety of garnet. II. Pyrope; Magnesium aluminium garnet, Mg 3 AJ 2 (Si0 4 )3 ; MgO = 29.82, A1 2 3 = 25.40, SiO 2 = 44.78; H. = 7.5; G. = 3.7 ; Color, dark red to nearly black. This is the precious garnet of the jewelers, often called the " Cape Ruby." Good pyropes are associated with serpentine at 444 MINERALOGY Bilin, Bohemia; pyrope is found in the "Blue Ground " with the diamond in South Africa ; in New Mexico on the Navaj o Reser- vation, where it is associated with chrysolite. III. Almandite; Common Garnet; Iron aluminium garnet, Fe 3 Al 2 (Si0 4 )3; FeO = 43.34, A1 2 3 = 20.34. SiO 2 = 36.15; but composition variable ; Color, red to black; Transparent to opaque. The clear red speci- mens are the " carbuncle," used as a precious garnet. Large crystals of this garnet are of common occurrence, as at Salida, Colorado; Fort Wrangle, Alaska, where sym- metrical combinations of the dodecahedron and the tetrag- onal trisoctahedrons are ob- tained. Dodecahedrons of an inch in diameter are found in a schist at Southbury, Con- necticut. Crystals three or four inches in diameter occur at Arendal, and somewhat smaller specimens at Bodo, Norway. All the well-developed and large crystals are in a mica schist. IV. Spessartite; Manganese aluminium garnet, Mn 3 Al 2 (Si0 4 ) 3 ; MnO = 42.95, A1 2 O 3 = 20.73, SiO 2 = 36.30 ; H. = 6.5 ; G. = 3.8 ; Color, dark hyacinth-red to brown-red. Beautiful specimens of this garnet are found at Amelia Court House, Virginia ; also at Salem, North Carolina ; Haddam, Connecticut ; and Bethel, Maine. V. Andradite ; Calcium iron garnet, Ca 3 Fe 2 (SiO 4 ) 3 ; CaO = 33.06, Fe 2 O 3 = 31.49, SiO 2 = 35.45; in the black melanite and schorlomite some of the silica may be replaced by titanium; H. = 6.5 ; G. = 3.8 ; Color, various shades of green and yellow to black. Demantoid is a massive green variety often polished as an ornamental stone. Aplome is also a green variety found at Schwarzenberg, Saxony. Topazolite is greenish yellow and found at Ala in Piedmont. Polyadelphite is a brown variety found in large crystals and massive at Franklin, New Jersey; it contains considerable manganese. Black varieties are also found at Frank- lin, while the titaniferous garnets occur at Magnet Cove, Arkan- sas; and at Henderson, North Carolina. FIG. 484. Garnet. SILICATES, TITANATES, ETC. 445 VI. Uvarovite : Calcium chrome garnet, Ca 3 Cr 2 (Si0 4 ) ; CaO = 29.27, Cr 2 O 3 = 32.50, SiO 2 = 38.23 ; H. = 7 ; G. = 3.4; Color, bright green. Fuses with difficulty and will not gelatinize after fusion. This garnet is usually associated with chromite and with serpentine ; at Oxford, Canada, however, it is found in the cavities of a granular limestone. It occurs at Wood's chrome mine, Lan- caster, Pennsylvania ; at New Idria, California. In rock sections garnet appears in crystalline outline, granular or irregular, colorless or in pale colors, with a high relief and a rough surface and parting at times distinct. A zonal structure FIG. 485. Section of Garnet, showing the High Relief and Parting. is often noticed, especially in the titaniferous varieties. Isotropic, but may exhibit anomalous weak double refraction, which is very often a characteristic of the garnets of contact zones. Occurrence. Garnets occur as accessory minerals in rocks of all varieties, the kind depending upon the nature of the magma. Andradite and almandite are found in granites; pyrope is con- nected with peridotites and serpentine; spessartite is found in quartzite and rhyolite; grossularite is the common garnet of crys- talline limestone; while all may be found in crystalline schists and gneisses, as well as in metamorphic and contact zones. Eclogite is a rock composed almost entirely of massive garnet. Garnet is easily decomposed by weathering, and forms chlorite, iron ores, calcite, kaolinite, epidote, and a large number of second- ary minerals, depending upon the chemical composition of the original garnet. Garnets when fused break down, and the melt on cooling forms other silicates, as anorthite, pyroxenes, or scapo- lite. They are therefore unstable at the temperature of fusion. Some garnets have been formed, as spessartite, by a simple fusion 446 MINERALOGY of the constituent oxides ; but as a rule some flux, as calcium chloride, must be added to lower the fusing point to a tempera- ture at which the formation of garnets is possible. OLIVINE GROUP The olivine group is composed of isomorphous orthosilicates of the general formula R" 2 SiO 4 , in which R" is Ca, Mg, Fe", Mn, Zn, or mixtures of these metals ; they are of orthorhombic symmetry and members of the didigonal equatorial type, crystallizing usually in combinations of the three pinacoids with the unit pyramid and a dome, or pyramidal in habit; at times tabular, parallel to b more often than to a. They have two well-developed cleavages at a right angle. The following table will serve to show their relations both opti- cally and chemically. OLIVINE Olivine. Chrysolite ; Magnesium iron orthosilicate ; MgFe- Si0 4 ; MgO = 49.19, FeO = 10.54, SiO 2 = 39.85 ; a : b : c = .4656 : 1:.5865; 1 10 A 100 = 24 58'; 001 A 101 = 51 33'; 001 A 011 = 30 24' ; Common forms, a (100), b (010) ; c (001), m (110). s (120), d(101), e(lll), k(021); Twinning plane, Oil rare; Cleavage, b distinct, a less so ; H. = 6.5-7 ; G. = 3.27-3.37 ; Color, shades of green to red or brown ; Streak, white or pale ; Luster, vitreous ; Transparent to opaque; a = 1.653; p = 1.670; -y = 1.689; y-a = .036 ; Optically () ; Axial plane = 001 ; Bx a = or b ; 2 V = 86 89'. B.B. Dark-colored specimens fuse to a magnetic slag, while the light-colored specimens whiten and fuse with difficulty. Gelat- inizes with HC1. With the fluxes reacts for iron. General description. Crystals are usually small and nearly equidimensional or tabular, parallel to a or b ; more often granu- lar, friable masses, in which form it is often found in large rock areas, as the dunites of Georgia, North and South Carolinas, where it is associated with corundum. Chrysolite and peridote are names often applied to olivine, but more particularly to the clear transparent varieties, which are used as gem stones. Peridote is leaf-green in color, and for a long time was gathered along the shore of the Red Sea, where the water-worn pebbles were thrown up by SILICATES, TITANATES, ETC. 447 1 O H O i Sao ^ ^ ^ ^ ^ ^ S-, SJ 8 8 2 8 8 88 v ' v^ r v l>0 = 4 ,O >etf *! IX> If0 UH.0,.0 o o 00 00 00 1 II II II II II II 7 II II II II 1! II a d co. v y ? CO. d oo. ?- d CQ. ? d CQ. ?- 00 05 iO CO O iO g CO 00 oo 8 iO O iO iO iO >O o .. s? i 8 CO *o S i g , ^ ^ ^ . . . IE T 1 ifl ^ 1 CO 'N ^N 55 1 1 (M CO 05 1 ( 1 1 rH O w W CO CO co CO Tfl T}H T}H T^ coO c5 ^ o COMPOSITION CaMgSiO 4 6 OQ CaMnSiO 4 (MgFe) 2 SiO (FeMgMn) 2 Si 9 .a o ^9^ a ? i N a? fe a pj ^ |SS GO Monticellite . . Forsterite . . . Glaucochroite . . Olivine .... Hortonolite. . . Fayalite .... Knebelite . . . Tephroite . Roepperite . . . 448 MINERALOGY the waves after storms. It was also obtained from the Arabs of Egypt, but the exact locality from whence it was gotten is unknown. Gem material is found near Fort Defiance, Arizona. Chrysolite is a yellow variety resembling very much the yellow topaz when cut and polished. Olivine occurs as an essential constituent of many igneous rocks, which are low in silica and rich in the alkali earth metals. Its composition is variable, depending upon the proportion of mag- nesium, calcium, iron, or manganese, present in the magma. It is found in such rocks" as peridotite, gabbros, basalts, nephelites, and leucites, and many lavas, while dunite is almost entirely olivine. It appears less often in andesites and trachytes. FIG. 486. Olivine Crystals from France. In rock sections it is colorless or pale, with crystalline outline, or more often granular or irregular. The two cleavage directions are well marked, that parallel to b more so than that parallel to a. Extinction parallel. The index of refraction being high, the relief is marked, with all cracks distinct. Inter- ference colors of the second and third order. The plane of the optic axes is parallel to 001 and the optical character is plus, with the acute bisectrix a, when the ferrous oxide is below 12 per cent. ; when above 12 per cent., the acute bisectrix is b and the optical character is minus. Inclusions are not characteristic, but spinel, chromite, apatite, and hypersthene appear ; also glass and slag in the lava occurrences. Pleochroism is marked only when the iron content is high. In its alteration olivine readily forms serpentine, the alteration following the fractures or cleavage cracks in the crystal, with the serpentine fibers lying crosswise. The iron at the same time separates as oxide and is deposited along the cracks, or where the specimen is rich in iron as layers interlaminated with the serpen- tine; carbonates, as magnesite and calcite, and also opal, quartz, SILICATES, TITANATES, ETC. 449 and brucite may appear as alteration products. Chromite is almost always associated with the alteration of peridotite or with the serpentine resulting from it, as at the Maryland locality. FIG. 487. Section of Olivine, showing the Cracks filled with Secondary Mag- netite and a Marginal Band of Enstatite and Hornblende. Some nickel deposits, as the garnierite of New Caledonia, are asso- ciated with olivine or its alteration products, as also platinum and the diamonds of South Africa. Artificially the olivine group is easily synthesized by a direct fusion of their constituents in the right proportion, particularly if a little boric acid is added to lower the fusing point. They are therefore common products of slags and are also found in meteors. When olivine is fused with a little silica in excess, enstatite is formed; if the silica is increased, then pyroxenes are formed. MONTICELLITE Monticellite. Calcium magnesium orthosilicate, CaMgSi0 4 ; CaO = 35.9, MgO = 25.6, Si0 2 = 38.5 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = .4337 ; 1 : .5758 ; 100 A 110 = 23 27'; 001 A 101 =53*; 001 A 011 = 29 56'; Common forms, as in olivine ; Cleavage, b distinct ; Brittle ; Fracture, conchoidal ; H. = 5-5.5; G. = 3.03-3.25 ; Color, white, gray; Streak, white ; Luster, vitreous; Transparent to translucent; a = 1.650; p = 1.662; -y = 1.668 -y - a = .018; Optically (-); Axial plane 001 ; Bx a = b ; 2 V = 37. 2G 450 MINERALOGY B.B. Fuses on the thin edges. Gelatinizes with HC1 ; this solution freed of silica yields little or no precipitate with ammonia, but a heavy white precipitate with ammonium carbonate. General description. In crystalline habit like olivine ; also granular and in cleavable masses. It is a product of metamorphism, and as such occurs in ejected blocks of limestone at Monte Somma, Vesuvius. Large crystals nearly an inch in length occur at Mag- net Cove, Arkansas. In all other respects it agrees with olivine. though much more restricted in its occurrence. FORSTERITE Forsterite. Magnesium orthosilicate ; Mg 2 SiO 4 ; MgO = 57.00, SiO 2 = 42.9 ; Orthorhombic ; Type, Didigonal Equatorial ; a: b:c = .4666:1: .5868; 100 A 110 = 24 55'; 001 A 101 =51 34'; 001 A 011 = 30 21'; Common forms as in olivine; Cleav- age, b distinct, c less so ; H. = 6-7 ; G. = 3.21-3.33 ; Color, white and light shades of yellow to green ; Streak, white ; Luster, vitreous; Brittle; Fracture, conchoidal; Transparent to trans- lucent ; p = 1.659 ; y - a high ; Plane of the optic axes = 001 ; Bx a = a; 2V = 86. B.B. Infusible, gelatinizes with HCl, this solution freed of silica, an excess of ammonia added yields no precipitate with am- monium carbonate (calcium), but yields a white precipitate with ammonium phosphate (magnesium). General description. In crystalline habit like olivine, but is largely a product of metamorphism ; as such it occurs in the ejected blocks of limestone at Monte Somma, Vesuvius. A variety, Bol- tonite, occurs as embedded crystals and disseminated grains in a limestone at Bolton, Massachusetts. FAYALITE Fayalite. Ferrous orthosilicate ; Fe 2 SiO 4 ; FeO = 70.6, Si0 2 = 29.4 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = .4584 : 1 : .5793 ; 100 A 110 = 24 38' ; 001 A 101 = 51 39' ; 001 A Oil = 30 5'; Forms as in olivine; Cleavage, b distinct, a less so; Brittle; Fracture uneven; H. = 6.5; G. = 4-4.14; Color, yellow to nearly black; Streak, pale when unoxidized; Transparent to opaque; a = 1.824; p = 1.864; -y = 1.874; SILICATES, TITANATES, ETC. 451 Y - a = .050; Optically (-) ; Axial plane = 001 ; Bx a = b ; 2 V = 49 50'. B.B. Fuses easily in the O. F. and becomes magnetic on coal in R. F. Gelatinizes with HC1. General description. Fayalite occurs in small crystals, flat- tened parallel to the orthopinacoid, in the rhyolites and ob- sidian of the Yellowstone Park. It has also been identified in a granite at Rockport, Mass. Originally it was described from Fayal Island, Azores. It is also a common constituent of furnace slags. It is easily decomposed by weathering, the iron being oxidized to ferric oxide. This takes place along the cracks and on the surface, the crystals becoming dark brown or black and at times iridescent. Knebelite is a massive mineral containing manganese, or may be considered as a mixture of fayalite and tephroite. Tephroite is the manganese orthosilicate, Mn 2 SiO 4 member of the olivine group. It occurs as a brown or red massive mineral at Franklin, New Jersey, where it is associated with zincite, wil- lemite, and franklinite. Roepperite is a variety from the same locality containing some zinc. WILLEMITE Willemite. Zinc orthosilicate ; Zn 2 Si0 4 ; ZnO = 73, Si0 2 = 27 ; Hexagonal ; Type, Hexagonal Alternating ; c = .6775 ; 0001 A lOfl = 38 2' ; l_0ll A I011 = 64 30'; Common forms, c(0001), a (1120), r(1011), e (0112) ; Cleavage, basal and pris- matic easy; Brittle; Fracture, conchoidal; H. = 5.5; G. = 3.89 -4.18; Color, shades of green, yellow, brown, and red; Streak, white or pale; Transparent to translucent; to = 1.6931; = 1.7118; -co = .0187; Optically (+). B.B. Whitens and fuses with difficulty. On coal in R.F. with soda and a little coal dust yields a zinc coat. Gelatinizes with HC1. General description. Crystals are hexagonal, prismatic in habit, terminated by rhombohedrons. It occurs at Franklin, New Jersey, in stout prisms an inch across and several inches in length ; these crystals contain manganese, and are red or ash color to dark 452 MINERALOGY brown, known as troostite. In all other localities the crystals are very small, as at Altenberg, Saxony, where the crystals are only a few millimeters in length. At Franklin it occurs in sufficient quantities to form a valuable ore of zinc, but mostly in cleavable yellow or light green masses, associated with zincite, franklinite, and tephroite. At times it forms long needle-like crystals, apple-green or nearly white in color, or in radiated masses; this last form is rare. The light colored varieties phosphoresce under the influence of magne- sium light or the radiations of radium, especially the radiated variety. Willemite has often been observed in furnace slags, where a lead ore containing zinc is being smelted. PHENACITE Phenacite. Beryllium orthosilicate ; Be 2 Si0 4 ; BeO = 45.55, SiO 2 = 54.45; Hexagonal; Type, Hexagonal Alternating; c = .6611j 0001 A lOll = 37 21' ; r A i f = 63 24' ; Common forms, a (1120), m(1010), r(1011), x (1232) ; Cleavage, a indistinct, r imperfect ; Brittle ; Fracture, conchoidal ; H. = 5-8 ; G. = 2.97- 3 ; Color, white, pale yellow, brown, or red ; Streak, white ; Luster, vitreous; Transparent to translucent; o> = 1.6542; = 1.6700; -a= .0158; Optically (+). B.B. Infusible, with borax in fine powder fuses to a clear glass. With cobalt solution yields a dull blue. Yields reactions for beryllium. General description. Crystals are rhombohedral in habit or short prismatic, terminated by rhombohedrons, at times, of all three orders. In its occurrences it is associated with beryl and topaz. The largest crystals, some nearly four inches across, are obtained near Ekaterinberg in the Urals, Russia. At Pike's Peak it is found implanted on crystals of microcline. Phenacite when clear and free of flaws is polished as a gem stone. DIOPTASE Dioptase. An acid copper orthosilicate, H 2 CuSi0 4 ; CuO = 50.4, SiO 2 = 38.2, H 2 O = 1 1 .4 ; Hexagonal ; Type, Hexagonal Alternating; c = .5341; 0001 A 1011 = 31 40'; r A r' = 54 5'; SILICATES, TITANATES, ETC. 453 Twinning plane 1011 = Common forms, a (1120), r (HH1), s (0221). Cleavage, rhombohedral perfect; Brittle; Fracture, conchoidal; H. = 5; G. = 3.28-3.35; Color, emerald green; Streak, pale green; Luster, vitreous ; Transparent to translucent ; = 1.562; = 1.546; CD = .016; Optically (-). B.B. Fuses easily with intumescence to a blebby glass. The fused mass powdered gelatinizes with HC1. General description. Crystals coarse prismatic, combination of the two prisms a and m terminated by the unit pyramid or the two unit pyramids. The third order prism h (210) and the pyra- mid z (311) occur on crystals from Grass Lake, New York. Wernerite is often granular or massive. In sections the scapolites are colorless, either in crystalline outline or rounded grains ; pris- matic cleavage cracks distinct; relief is very low, about that of quartz. The inclusions are not characteristic ; double refraction rather strong, yielding interfer- ence colors of the second order and increasing with the amount of calcium present. The inter- ference figure is found in the sections in which the cleavage cracks are at right angles, yielding a dark cross with color bands in thin sections, in the margin of the field only. Optically negative. The scapolites are found in igneous rocks as secondary minerals only. They are the products especially of contact metamorphism and commonly occur in granular limestones, where they are associated with pryroxene, hornblende, zircon, spinel, titanite, and garnets. Wernerite occurs in fine crystals at Pierrepont, Gouverneur, Monroe, and Amity, New York, and at numerous points in New Jersey, Pennsylvania and the New England States. Meionite and mizzonite occur in the ejected blocks of limestone on Monte Somma, Vesuvius. FIG. 488. Wernerite. Laueinpaei, Finland. SILICATES, TITANATES, ETC. 455 The scapolites are easily decomposed by weathering, particularly those containing sodium, the ultimate products being kaolin, talc, or micas, with calcite and quartz. The artificial production of the scapolites is in doubt. VESUVIANITE Vesuvianite. An orthosilicate of calcium and aluminium, Ca 6 (Al . OH)Al 2 (Si0 4 ) 5 , in which other isomorphous elements may enter; CaO = 42.3, A1 2 = 19.1, SiO 2 = 37.5, H 2 = 1.1; Tetragonal; Type, Didigonal Equatorial; c = .5372; 001 A 101 = 28 15'; 001 A 111 = 37 14'; Common forms, c (001), a (100), m(110), p (111) ; Cleavage, prismatic imperfect, a and c less so; Brittle ; Fracture, uneven ; H. = 6.5 ; G. = 3.35-3.45 ; Color, shades of brown and green ; Streak, white ; Translucent ; CD = 1.705; = 1.701; o> - = .004; Optically (-), sometimes (+). B.B. Fuses with intumescence at three, to a brown slag, which when powdered gelatinizes with HC1. Some varieties will yield reactions for manganese or copper. General description. Crystals are well developed, stout pris- matic combinations of the prisms of the first and second orders terminated by the base and the unit pyramid. Striations on the prism zone lengthwise. Beautiful crystals, combinations of these forms, are found on the Vilui River, Siberia. Crystals from Wakefield, Quebec, have the pryamid reduced to almost a line and are terminated by the base. Well-formed crystals, combinations of all seven forms of the type, are found at Poland, Maine, and more complicated combinations are found on the crystals from Vesuvius, in some of which the prism zone is very much reduced, yielding crystals of pyramidal habit. Vesuvianite also occurs radiated, columnar, irregular, granular or massive. A compact, green, jade- like variety is known as californite. Chemically the calcium may be replaced in part by manganese, magnesium, or iron ; also fluorine and boron may be present in small amounts. In sections Vesuvianite may appear in crystalline out- lines, rounded or irregular, colorless or pale. The relief is well marked, with prismatic cleavage cracks imperfectly developed. Pleochroism faint, but increasing with the depth of color of the specimen. Interference colors gray of the first order. Basal sections show only the shadow of the interference figure but no colors. Optically negative, rarely positive. 456 MINERALOGY Zonal structure and optical anomalies are not uncommon. Vesuvianite is a mineral produced by contact metamorphism. It appears in schists but more often in granular limestones, where it is associated with garnets, epidote,wer- nerite, wollastonite, and diopside. It is a common mineral in the ejected blocks of limestone on Monte Somma, Ve- suvius ; in small bril- liant crystals also in the Ala thai, Pied- mont. Clear green brilliant crystals oc- cur at Amity, New York. It occurs at Rumford and Po- land, Maine, in lime- stones associated with garnets; at Newton, New Jersey, with corundum and spinel. The clear crystals are sometimes polished, but it makes an indifferent gem known as idocrase. It is seldom found altered, though pseudomorphs after vesuvian- ite are known. Vesuvianite has not been produced artifically; when fused it breaks down and on cooling the melt produces olivine, anorthite, and melilite. FIG. 489. Vesuvianite, Poland, Maine. The Upper Crystal is Viluite from Siberia. ZIRCON Zircon. Zirconium orthosilicate, ZrSiO 4 ; ZrO = 67.2 ; SiO 2 = 32.8 ; Tetragonal ; Type, Ditetragonal Equatorial ; c = .6493 ; 001 A 101 = 32 38'; 11(^111=47 50'; 111 A 111=56 40'; Common forms, p(lll), m(110), u(331), x (311) ; Twinning plane 111, geniculate twins; Cleavage, m distinct, p less so; Brittle ; Fracture, conchoidal ; H. = 7.5 ; G. = 4.68-4.7 ; Color, SILICATES, TITANATES, ETC. 457 pale yellow, brown, red, green, white, black. Streak, white ; Transparent to = 1.968; co - = .045; Optically (+). and at times nearly opaque; co = 1.923; B.B. Whitens but infusible. Only very slightly affected by acids. Yields a zirconium reaction with turmeric paper. General description. Crystals are short prismatic combinations of the unit prism and pyramid of the first order. When x is present the crystals are acutely pointed. Microcrystals are more apt to be pyramidal in habit. Twins are not common, but genicu- late twins like those found in rutile occur in Renfrew County, Quebec. Zircon is related in its angles and axial ratio to cassiterite and rutile, and these with thorite, ThSiO4, constitute an isomorphous group. In rock sections zircon appears either in crystalline outlines or rounded, with a very high relief, and white or pale yellow or brown in color. Cleavage cracks are not marked. Interference colors are high fourth order, and the interference figure shows several colored circles in addition to the dark cross. Optically positive. Zircon is very widely distributed, occurring as one of the most common accessory minerals of the igneous rocks, as the granites, syenites, and diorites, but never in very large quantities ; in such magmas it is the first silicate to separate. It is also of common occurrence in pegmatites, as at Green River, North Carolina, where it is separated in commercial quantities; near Cash, Okla- homa, in a pegmatite. At Greenville, Canada, and Amity, New York, it occurs in a crystalline limestone. At several points in Essex and Orange Counties, New York, deep brown to almost black crystals occur. It is decomposed by weathering with difficulty and is found in alluvial deposits and gold-bearing sands, with garnets, cassiterite, magnetite, and other heavy minerals, still in a fresh unaltered FIG. 490. Zircon, Buncombe County, North Carolina. The Small Crystal is from Essex County, New York. 458 MINERALOGY condition. At times by hydration it becomes dull and greasy in appearance, forming several varieties, as the malacon from Hit- teroe, Norway. Jargon, jacinth, and hyacinth are clear varieties usually obtained from Ceylon, which are cut and polished as gems. Artificial zircon may be produced by heating gelatinous silica and zirconia to a red heat under pressure. TOPAZ Topaz. Al . Al(O,F 2 )Si0 4 ; A1 2 O 3 = 55.44, Si0 2 = 32.61, F = 20.65 ; Orthorhombic ; Type, Didigonal Equatorial ; & : b : c = .528; 1 : .477; 110 A 110 = 55 43'; 120 A 120 = 86 49'; 001 A lll = 45 35'; 001,223 = 34 14'; 001 A 221 = 63 54'; 001 A 041 = 62 20'; 001 A 043 = 32 27'; 001 A 201 = 61 30"; 001 A 021 = 43 39'; Common forms, a (100), b (010), c(001), m(110), 1(120), d(201), f(021), y(041), x(043), o(221), u(lll), i (223) ; Cleavage basal perfect ; Brittle ; Fracture, subconchoidal ; H. = 8 ; G. = 3.4-3.65 ; Color, white and pale shades of yellow, pink, blue, green.; Streak, white ; Transparent to translucent ; a = 1.615 ; P = 1.618; 7 = 1.625; V- a = -010; 2V = 62 33'; Axial plane = 010; Bx a = c; Optically (+). B.B. Changes color, but infusible. Yields a fluorine reaction in the closed tube. The powdered mineral becomes blue with cobalt solution. Very little affected by acids. General description. Crystals are prismatic in habit, combi- nations of the two prism, m and 1, both of which are striated length- wise, but 1 more than m ; terminated by one or two domes and pyramids, rarely doubly terminated. In chemical composition the fluorine is variable, as it may be replaced by hydroxyl (OH) ; both of these are driven off at a white heat ; transforming the topaz to sillimanite. In rock sections topaz appears with crystalline outlines or lath- shaped, elongated parallel to 6 ; the basal cleavage is well marked by cracks. Relief is medium and the interference colors are first order gray or yellow. The interference figure is shown in the basal section ; the angle 2 E varies with the fluorine and decreases as OH increases. For .93 per cent. H 2 0, 2E = 114. Cavities elongated parallel to the vertical axis and containing fluids are common. Topaz is an accessory mineral in many granites, and is also SILICATES, TITANATES, ETC. 459 especially characteristic of pegmatites containing cassiterite, in which it is also associated with beryl, tourmaline, fluorite, and apa- tite. At Schneckenstein, Saxony, it is associated with apatite chalcopyrite, and cassiterite. The large blue crystals from Mursinka, in the Urals, are asso- ciated with smoky quartz, lepidolite, and feldspars. At Nathrop, Colorado, and in the Thomas Range, Utah, well-developed crystals, both white and wine-colored, are associated with quartz, in cavi- , ties in rhyolite. At Stoneham, Maine, it occurs in granite. The topazes of Minas Geraes, Brazil, occur in a decomposed schist ; they are a light brown color with numerous elongated cavities, containing liquid carbon dioxide. Topaz, owing to its hardness, transparency, and delicate coloring, has for a long time been used as a precious stone, especially those from Siberia and Brazil. The Brazilian topazes may be improved in color and the yellow and brown shades changed to a delicate pink by careful heating ; this process is known as pinking. In the process of weathering, topaz takes up water and alkalies, forming micas. The synthesis of topaz has been accomplished by heating a mix- ture of silica and aluminium fluoride to a red heat and then igniting the mixture in a current of silicon fluoride. Both the synthesis and the associations indicate that in many cases topaz has been the result of pneumatolytic reactions in which volatile fluorides were the direct agent. ANDALUSITE Andalusite. Al(AlO)SiO 4 ; A1 2 O 3 = 63, Si0 2 = 37; Ortho- rhombic ; Type, Didigonal Equatorial ; fi : b : c = .986 : 1 : .702 ; 100 A 110 = 44 36'; 001 A 011 = 35 5'; Common forms, c (001), m (110), s (Oil) ; Cleavage, prismatic distinct, a less so; Brittle; Fracture, uneven ; H. = 7.5; G. = 3.16-3.20; Color, gray, reddish, pink, blue, and green; Streak, white; Luster, vitreous; Trans- parent to opaque; a = 1.632; p = 1.638; \ = 1.643; -y-a = .011; Optically (-); Bx a = c; 2V = 83 37'. B.B. Infusible, not attacked by acids. The fine powder becbmes blue with cobalt solution. General description. Crystals are coarse, prismatic, simple combinations of the nearly square unit prism and the base or dome ; 460 MINERALOGY FIG. 491.'' Andalusite from Lancaster, Massa- chusetts. such combinations are found at Lisens Alp, Tyrol, embedded in a chloritic schist. Transparent crystals which display very strong dichroism are obtained in Minas Geraes, Bra- zil, and are cut as gem stones showing green when viewed along one direction and red when viewed in the other. Many specimens of andalusite contain or- ganic inclusions ar- ranged symmetrically, the outline or cross section of which varies with the position in the crystal. Like the symmetrical inclu- sions in leucite, this arrangement is prob- ably due to a skeletal development during the growth of the crystal ; such inclusions are especially characteristic of the variety known as chiastolite, found in argillaceous schists and clay slates, the crystals of which are slender, prismatic, almost acicular. Andalusite is trimorphic with sillimanite and cyanite, all being of the same percentage composition chemically, but differing in their physical and crystallographical properties. Of the three, silli- manite is the most stable at high temperatures, as both andalusite and cyanite when heated to 1400 C. pass over to sillimanite on cooling. In rock sections andalusite appears in almost square or in elon- gated outlines. Relief is marked, and the pleochroism shows only in the colored varieties. Inclusions are symmetrically arranged. Interference colors are yellows of the first order. Andalusite is the result of metamorphism and is developed in some gneisses and schists, where it is associated with sillimanite, cyanite, iolite, garnets, corundum, and tourmalines. Specimens with typical inclusions are found at Lancaster, Massachusetts, and Rochester, New Hampshire. It is a common mineral at numerous points in New England. SILICATES, TITANATES, ETC. 461 The name andalusite is derived from the noted locality of Anda- lusia, in Spain. Andalusite in weathering is decomposed by percolating waters containing alkalies, forming micas and kaolinite. SILLIMANITE Sillimanite. Al(AlO)SiO 4 ; A10 3 =_63 ; Si0 2 = 37 ; Ortho- rhombic ; a : b : c = .970 : 1 : ? 110 A 110 = 88 15' ; 230 A 230 = 69; Common forms, a (110), b (010), m(110), h (230) ; Cleavage, b perfect; Brittle; Fracture, uneven; H. = 6-7; G. = 3.23-3.24; Color, shades of gray, brown, and green ; Streak, white; Luster, vitreous; Transparent to opaque; a = 1.660; p = 1.661 ; -y = 1.681 ; y - a = .020; Axial plane = 010; Bx a = c; Optically (+); 2V = 31 19'. B.B. Like andalusite. General description. Crystals are slender, elongated, parallel to the vertical axis ; from this habit the mineral is sometimes known as fibrolite. Terminations have never been observed. Also in compactly massed fibers arranged parallel or radiating. In sections sillimanite may be distinguished from andalusite by the fibrous structure, by the higher double refraction yielding interference colors of the second order, and by the optical character being posi- tive. In occurrence and decomposition by weathering, it resembles andalusite. CYANITE Cyanite. Disthene, Al(AlO)SiO 4 ; _A1 2 O 3 = 63, Si0 2 = 37 ; Triclinic ; Type, Centro-symmetric ; a : b : c = 0.8994 : 1 : .7089 ; a = 90 5' ; (3 = 101 2' ; \ = 105 44' ; 100 A 010 = 73 56' ; 100 A 001 = 78 30'; 010 A 001 = 86 45'; 100 A 110 = 34 17'; Common forms, a (100), b (010), c (001), m(110); Twinning plane, 100 ; Cleavage, a perfect, b less so and parting parallel to c ; Brittle; Fracture, fibrous; H. = 5-7.25; G. = 3.56-3.67; Color, blue, white, reddish, and green; Luster, vitreous; Transparent to translucent; a = 1.717; p = 1.722; 8 3 O o oo o oo & 1 CO O5 l> 00 u + 1 -H -H d CO O O O oq 1 8 q q S8 b % C O cog g CS| (M CO C^ (N CO 00 g CO CO !> J 3E II II II II II II II II II II IBS "w d co. ?- d GO. ?- d GO. ?- CO. O5 CO CO rH CO (N Oi CO CO rH CO rH O rH JD ^ o o o C8 8 II I s " IIO II Ci ro 00 rn C fp O q o co >o rH rH rH co 00 rH iO CO *O CO CO CO CO CO O S 2 .? ffi o fe s S ^ ^ O o3 ^ > ' 5 M 3 w 2 ^ ^ ^ ^ ^ fe ^fe a 1 I o 3 1 1 *5,* < o ^ ^ s CO CO 7"\ o o S 3 * D OJ t3 .^ 1 S a 1 J 1 1 N I I ^ 1 SILICATES, TITANATES, ETC. 465 with the increase of iron and manganese in the formula they become monoclinic. ZOISITE Zoisite. A basic orthosilicate of calcium and aluminium ; Ca 2 Al 2 (A10H) (SiO) 3 ; CaO = 24.6, A1 2 O 3 = 33.7, SiO 2 = 39.7, H 2 O = 2.0 ; Orthorhombic ; Type, Didigonal Equatorial ; & : b: c = .6196: 1 : .3429; 100 A 110 = 31 47'; 001 A 101 = 28 58'; 001 A 011 = 18 56'; Common forms, a (100), b(010), m(110), o(lll), d(101), f(011), r(120); Cleavage, b perfect; Brittle; Fracture, uneven; H. = 6-6.5; G. = 3.25-3.37; Color, gray, white, or pale shades of green, pink, red, or yellow ; Streak, white ; Luster, vitreous to pearly; a = 1.696; p = 1.696; 7 = 1.702; Y a = .006 ; Optically ( + ) ; Axial plane = 010 at times 001 ; Bx a = a; 2V = 0-60. B.B. Fuses with intumescence at three to a white blebby slag, gelatinizes after fusion. After strong ignition in the closed tube yields water. General description. Crystals are prismatic in habit, elon- gated parallel to the vertical axis, while epidote is elongated paral- lel to the orthoaxis ; in the comparison of these two minerals the c axis of zoisite is equivalent to the b axis of epidote. Deep stria- tions on the prism zone parallel to the vertical axis are characteristic. The crystals are rarely terminated, occurring in parallel or diver- gent groups ; also massive. Thulite is a pink variety from Norway, and Traversella, in Pied- mont ; the pink color is due to manganese. Zoisite contains but little iron and is essentially an aluminium epidote. Clinozoisite is a light-colored variety of epidote with small amounts of iron ; the characteristic pistachio-green color of epidote deepens with the increase of iron in the molecule. In rock sections zoisite appears in elongated crystals or granu- lar; colorless, or pale; with the cleavage cracks parallel to the macropinacoid distinctly developed. Relief is high, but the interference color is a low first order gray. Extinction parallel. The plane of the optic axis is usually parallel to the base, but at times may be parallel to the macropinacoid. Optically ( + ). Zoisite is associated with the crystalline schists ; rarely is it found in granites or igneous rocks. As a secondary mineral it is 2n 466 MINERALOGY derived from the alteration of the plagioclases. Saussurite is a mixture of plagioclase and zoisite in various proportions. It occurs in various localities in Massachusetts, Connecticut, and North Carolina ; at Ducktown, Tennessee ; and in the Coast Range, California. The synthesis of zoisite is uncertain, as the products of various fusions have contained no water. EPIDOTE Epidote. A basic orthosilicate of calcium aluminium and iron, Ca 2 (AlFe) 2 (AlOH)(Si0 4 ) 3 ; CaO = 23.73, A1 2 3 = 25.95, Fe 2 O 3 = 10.18 (when Fe : Al : : 4 : 1), SiO 2 = 35.20, H 2 O = 1.91 ; Mono- clinic ; Type, Digonal Equatorial ; a : b : c = 1.5787 : 1 : 1.8036 ; p = 64 37' = 100 A 001; 100 A 110 = 55; 001 A 101 = 34 43'; 001 A 011 = 58 28'; Common forms, c (001), a (100), m(110), e(101), r(101), o(011), d(lll); Twinning plane, 100 contact twins, also 001, but rare ; Cleavage, basal perfect and a imperfect ; Brittle ; Fracture, uneven ; H. = 6-7 ; G. = 3.25-3.5 ; Color, shades of green, also yellow, red, or gray ; Streak, white ; Luster, vitreous; Transparent to opaque; a = 1.724; p = 1.729; y = 1.734; -y - a = .010; Optically ( - ); Axial plane = 010; Bxa A c 2-3 in the acute angle p; 2V = 73-88. B.B. Fuses at three with intumescence to a black blebby glass, which when powdered is generally magnetic and gelatinizes with HC1. After strong ignition in the closed tube yields water. FIG. 493. Epidote, Sulzbachthal, Tyrol. The Central Figure is from Prince-of- Wales Island, Alaska. General description. Crystals are elongated parallel to the orthoaxis, with terminations generally rich in faces ; often twinned SILICATES, TITANATES, ETC. 467 as is indicated by the reentrant angle. Particularly fine speci- mens of this habit occur in a chloritic schist, associated with adu- laria, apatite, titanite, and calcite, in the Sulzbachthal, Tyrol. A tabular habit, though not as common as the elongated habit, also occurs, good specimens of which are obtained on the Prince of Wales Island, Alaska ; these are also twinned, the twinning being revealed by the striations on the clinopinacoidal face. Massive FIG. 494. Epidote. Sulzbachthal, Tyrol. and granular epidote mixed with quartz occurs as the rock epido- site; it is derived from the alteration of plagioclase feldspars together with some ferromagnesian mineral, as pyroxene or amphi- bole. Piedmontite is a brown or red epidote in which the iron is re- placed by manganese ; it occurs in Piedmont, Italy, and also in a rhyolite at South Mountain, Pennsylvania. In rock sections epidote appears colorless, pale yellow, or brown, depending upon the percentage of iron; in tabular or elongated crystals, at times intergrown with zoisite. The relief is high, and the basal cleavage cracks are distinct. Pleochroism is strong in the colored varieties, and much less in the colorless, or those poor in iron. Interference colors are high, as the double refraction may vary from .03 to .06. The extinction is inclined and varies from 2 468 MINERALOGY to 3 with the imperfect orthopinacoidal cleavage cracks. Basal cleavage fragments show one optic axis, while the acute bisectrix is nearly perpendicular to 100. Optically ( ). Epidote appears as a secondary mineral in igneous rocks and in schists, where it is derived from the alteration of feldspars and pyroxene or amphibole, usually associated with chlorite. It is also the product of metamorphism, and is found in contact zones, as well as in granular limestones where it is associated with vesu- vianite, garnets, hematite, and pyroxenes. It rarely occurs as a primary mineral of igneous rocks. In the United States epidote is a common mineral at various localities along the Atlantic slope in the New England, Middle, and Southern states. Its synthesis, like zoisite, is uncertain. ALLANITE Allanite. Ca(Al . Ce . Fe) 2 (A10H) (Si0 4 ) 3 ; Composition vari- able ; Monoclinic ; Type, Digonal Equatorial ; a : b : c = 1.5509 : 1:1.7691; p = 64 39' = 100 A 001 ; 100 A 110 = 5434'; 001 A 101 = 63 24'; 001 A 00:L = 58 3' ; Common forms, c (001), a (100)", m (110), d(lll), n(lll); Twinning plane, 100; Cleavage, 100 and 001 in traces ; Brittle ; Fracture, uneven ; H. = 5.5-6 ; G. = 3.5-4.2; Color, pitch brown to black or yellowish; Streak, pale gray or greenish; Luster, pitchy to dull; Opaque to subtranslu- cent. B.B. Fuses easily with intumescence. Becomes magnetic in R. F. After strong ignition in the closed tube yields water. Gelatinizes with HC1 ; the solution freed of silica yields reactions for cerium, page 571. General description. Either tabular in habit parallel to 100, or acicular parallel to the orthoaxis ; also granular or massive. The elongated variety has been described under the name of orthite ; it contains much water, even as high as 17 per cent., while the true allanite contains only one or two per cent. Allanite is an epidote in which some of the iron is replaced by the rare elements, cerium, lanthanum, didymium, yttrium, or erbium ; the amount of each varies with the locality ; the total of them all is about 20 per cent. Allanite occurs as an accessory mineral in igneous rocks, more often in those rich in silica, as granites and pegmatites; also in schists and crystalline limestones. SILICATES, TITANATES, ETC. 469 It is found at many localities along the Atlantic slope, as South Mountain, Pennsylvania ; Edenville, New York ; Haddam, Con- necticut ; Franklin, New Jersey ; Amelia Court House, Virginia ; Bethany Church, Iredell County, North Carolina. AXINITE Axinite. A borosilicate of calcium and aluminium, HCaa- Al 2 B(Si0 4 ) 4 ; Composition variable; Triclinic; Type, Cent'ro- symmetric; a : b : c = .4921 : 1 : .4797 ; a = 82 54'; p = 9152'; V = 131 32' ; 010 A 100 = 48 21/ ; 100 A 001 = 93 49' ; 010 A 001 = 97 50'; 110 A 100 = 15 34'; 110 A 100 = 28 53'; Common form, a (100), b (010), c (001), m (110), M (FlO), r (111), x (111), e (111) ; Cleavage 010 distinct, Brittle ; Fracture, conchoidal ; H. = 6.5- 7; G. = 3.27 - 3.29; Color, brown, blue, gray, or yellow; Streak, white or pale ; Transparent to translucent ; a = 1.685 ; p = 1.692 ; y = 1.695 ; -y a = .010 ; Optically ( ) ; Bx a nearly perpendicu- lar to 111; 2V = 71 38'. B.B. Fuses easily with intumescence, yielding a green flame (boron). Gelatinizes with HC1 after fusion, but insoluble before. May yield an iron or manganese reaction with the fluxes. FIG. 495. Axinite from Dauphine, France. General description. Crystals flattened, with the forms r, M, and m prominent, with edges sharp like an axe, hence the name 470 MINERALOGY axinite. Striations on the prism zone parallel to the vertical axis are characteristic. The color is usually clove-brown, but varies with the replacement of calcium with iron or manganese. Like most other borates, axinite is formed by pneumatolytic action, and therefore usually appears in the cracks and veins of granites and diabases, or in metamorphic contact zones. Fine crystals implanted on the walls of veins in diabase occur at Bourg d'Oisans, Dauphine', France; at St. Gothard, Switzer- land. In the United States it occurs a Franklin, New Jersey, in yellow crystals associated with garnets and rhodonite ; at Bethle- hem, Pennsylvania, and at Phippsburg, Maine. PREHNITE Prehnite. An orthosilicate of calcium and aluminium ; H 2 Ca 2 - Al 2 (SiO 4 ) 3 ; CaO = 27.1, A1 2 O 3 = 24.8, SiO 2 = 43.7,^ H 2 O = 4.4 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = .8401 : 1 : .5549 ; 100 A 110 = 40 2'; 001 A 101 = 33 27'; 001 A Oil = 29 2'; Common forms, c (001), a (100), b (010), m (110), o (061) ; Cleavage, basal distinct ; Brittle ; Fracture, uneven ; H. = 6-6.5 ; G. = 2.8-2.95 ; Color, light green, oil-green, yellow, white; Streak, white; Luster, vitreous; Nearly transparent to translucent; a = 1.616; p = 1.626; -y = 1.649; -y - a = .033; Optically ( + ) ; Axial plane = 010; Bx a = c ; 2 V = 69 22'. B.B. Fuses easily with intumescence to a blebby glass and gelatinizes after fusion with HC1. After the separation of silica , , and aluminium with ammonia, yields a heavy white pre- cipitate with am- monium carbonate (calcium) . Yields water in the closed tube. General descrip- tion. Crystals are small and rarely simple, but in par- FIG. 496. Prehnite. Bergen Hill, New Jersey. ., ... allel position or joined in ridged groups with a rough surface on which the indi- vidual crystals may be seen to be joined by the base, with the SILICATES, TITANATES, ETC. 471 prism angle free on the surface. Often globular or botryoidal with very small crystal faces. The nodules when broken show a radiated structure. The color is nearly always light green or yellowish, the color fading on exposure. Prehnite is a secondary mineral formed from solution in the cavi- ties and veins of the basic igneous rocks, where it is associated with the zeolites, datolite, calcite, and quartz. It is a common mineral in the cracks of the traps of Massachusetts, Connecticut, and New Jersey ; also in the Lake Superior copper regions. It forms pseudomorphs after analcite, natrolite, and the plagio- clases, and decomposes, forming chlorite. ? When fused it breaks down and on cooling yields wollastonite. CHONDRODITE Chondrodite. [Mg^. OH)] 2 Mg 3 (Si0 4 ) 2 ; Monoclinic ; Type, Digonal Equatorial ; a : b : c = 1.0863 : 1 : 3.1447 ; p = 90 = 100 A 001; 100 A 110 = 47 22';- 001 A 101 = 70 57'; 001,011 = 72 22'; Common forms, c(001), b (010), r 2 (125), r 3 (123) ; Twinning plane, 105; Cleavage, basal; Brittle; Fracture, conchoidal; H. = 6-6.5 ; G. = 3.1-3.2 ; Color, shades of yellow, brown, and red ; Streak, white or pale yellow; Luster, vitreous; Transparent to translucent; a = 1.607; p = 1.619; y = 1.639; I CO |>I CO i-H T-H O TJH co cq -^ 06 O b- iO O OS T-H 00 - = .004; Optically (-). B.B. Fuses with difficulty at 5. The powder moistened with H 2 S0 4 yields a green flame in O. F., or the nitric acid solu- tion shows phosphoric acid with ammonium molyb- date. The concentrated HN0 3 solution yields a white precipitate with H 2 S0 4 (Ca). General description. Crystals are stout prisms terminated with the pyra- mid of the first order, or with the pyramid in com- bination with the base. Combinations of the three orders of prisms, which as well as the etch figures fix the symmetry, occur on small, brilliant, colorless crystals found in a chloritic schist, associated with epidote and adularia, in the Untersulz- bachthal, Austria. All three orders of prisms are in combination on small tabular crystals, with small prism faces striated parallel to the vertical axis, occurring in veins associated with fluorite, FIG. 513. Apatite from Templeton, Canada. COLUMBATES, PHOSPHATES, VANADATES 509 cassiterite, and sulphides at Ehrenfriedersdorf, Saxony. Crystals rich in forms have also been described from Branchville, Con- necticut, and Alexander County, North Carolina. Chemically there are two compounds, a chlor-apatite and a fluor- apatite, which are isomorphous and occur in the same crystals. It is rarely that one occurs without the other ; in addition the chlorine or fluorine may be replaced by hydroxyl (OH) ; such specimens will yield a little water in the closed tube. Phosphatic rock found in the South and West is of the nature of apatite, but of organic origin ; a bone phosphate, phosphatic nodules, coprolites, all of which are FIG. 514. Apatite. Snarum, Norway. phosphates of calcium, but not crystalline, and therefore their com- position varies greatly. Extensive beds of these phosphates are found in South Carolina and the Gulf states ; after treatment with sulphuric acid they form the superphosphates of the fertilizer industry. Apatite occurs in rocks of all descriptions and under variable conditions. In igneous rocks it is always well crystallized, elon- gated parallel to the vertical axis ; one of the very first minerals to separate from the magma; it appears as inclusions in all others, even penetrating the magnetite. In rock sections it is colorless, with a hexagonal outline, or elon- gated when cut nearly parallel to c ; such sections usually show a tranverse parting, but the basal cleavage is seldom observed in sec- tions. The relief is well marked; interference colors are grays of the first order. Apatite is a common mineral in the metamorphic rocks and crystalline limestones, where it is associated with titanite, scap- 510 MINERALOGY olite, pyroxene, and vesuvianite. At Burgess, Ontario, hexagonal prisms a foot in length occur in the limestone. These large crystals always have a peculiar vitrified appearance, their edges are rounded as if they had been partly fused. Apatite occurs commonly along the Atlantic slope from Ontario to Georgia. It is associated with the tin veins of Bohemia and Cornwall, where its origin is due, as is also the cassiterite, to the chemical interaction of volatile fluorides and chlorides. It is very peculiar that apatite, being quite soluble in acids and a salt of a weak acid, decomposes in nature with difficulty. Under the action of percolating waters containing carbon dioxide the calcium phosphate passes into solution, to be again separated as various FIG. 515. Section of a Mica-diorite, showing a, Apatite; 6, Hornblende; c, Biotite ; e, Quartz ; and /, Feldspar partially Altered. secondary iron phosphates, as vivianite, Fe 3 (P0 4 )2 . 8 H 2 O, a common mineral of clays; also as dufrenite, Fe 2 (OH) 3 P0 4 ; phosphosiderite, 2 FeP0 4 . 3 H 2 ; strengite, FePO 4 . 2 H 2 ; as secondary aluminium phosphates, wavellite, A1 3 (OH) 3 (P0 4 ) 2 . 5 H 2 O; variscite, A1P0 4 . 2 H 2 ; turquoise, A1 2 (OH) 3 P0 4 . H 2 O. Artificial apatite has been formed by heating calcium and ammonium chlorides with calcium phosphate in a sealed tube at a temperature as low as 150 C. It has been reported as a con- stituent of some slags, but this has never been confirmed by analy- sis. Either chlor- or fluor-apatite may be produced in the dry fusion of sodium phosphate with either calcium chloride or calcium fluoride as the case requires. COLUMBATES, PHOSPHATES, VANADATES 511 PYROMORPHITE Pyromorphite. Pb 5 Cl(P0 4 ) 3 ; Chloro-phosphate of lead; PbO = 82.3, Cl = 2.6, P 2 5 = 15.7 ; Hexagonal ; Type, Hexagonal Equatorial; c = .7362; 0001 A 1011 = 40 22'; 0001 A 2021 =_59 32'; Common forms, c (0001), m(1010), x(lOfl), y (2021) ; Cleavage, m and x in traces; Brittle; Fracture, conchoidal; H. = 3.5-4 ; G. = 6.9-7 ; Color, shades of green, yellow, or brown ; Streak, white or pale ; Luster, resinous ; Subtranslucent to nearly opaque; o> = 1.51; = 1.45; co = .006; Optically ( ). B.B. Fuses easily at 3.5. With soda and borax in R. F. on coal yields lead buttons and a lead coat. Dissolves in HNO 3 , yields a yellow precipitate with ammonium molybdate. When a S. Ph. bead is saturated with copper oxide and heated with the powdered mineral it shows chlorine. Some specimens may con- tain arsenic. FIG. 516. Pyromorphite from Baumbach, Prussia. General description. Crystals are columnar, striated length- wise, usually hexagonal prisms roughly terminated or pitted at the termination. In rare cases they are terminated by the pyramid and base, as at Causthal in the Harz. Crystals from Ems, Nassau, are terminated by the base only. Also in parallel growths, irregu- lar aggregates, or granular ; sometimes in amorphous crusts and concretions. Pyromorphite is a secondary mineral formed by the interaction of water containing phosphates in solution and lead ores. It is 512 MINERALOGY characteristic of the zone of oxidation and is therefore found in the superficial workings of lead mines. While it is a valuable ore of lead, it occurs only in small quantities. Good specimens have been obtained at the Wheatley mine, Chester County, Pennsylvania. It occurs in small quantities at various localities in New England and North Carolina. Mimetite, Pb 5 Cl(AsO 4 ) 3 , a chlorarsenate of lead, is very similar in habit and occurrence to pyromorphite, with which it is iso- morphous. In color it is yellow to greenish white and often in globular or barrel-shaped crystals. Vanadinite, Pb 5 Cl(P0 4 ) 3 , a chlorvanadate of lead, another member of the apatite group, in which V 2 O 5 takes the place of P 2 O 5 or As 2 6 . It is similar in habit and crystallization, but usually red to light yellow in color. It is associated with lead ores at various localities in Arizona and New Mexico. Endlichite is a light yellow variety containing arsenic, occurring in Sierra County, New Mexico. AMBLYGONITE Amblygonite. Li(AlF)PO 4 ; Li 2 O = 10.1, A1 2 3 = 34.4, P 2 5 = 47.9, F = 12.9 ; Triclinic ; Type, Centrosymmetric ; a : b : c = .7334:1: .7633; a = 108 51'; p = 9748'; 7 = 106 27'; 100 A 010 = 69 25'; 100 A 001 =75 30'; 010 A 001 = 67 38'; 100AllO = 29 35' ; 001 A 021 = 74 40' ; Common forms, c (001), a (100), m (110), M(110), e(021); Twinning plane, 101 and 101, polysynthetic twins common, the two sets of striations making an angle of 89 8' ; Cleavage, basal perfect, at times e also; Brittle; Fracture, un- even; H. = 6; G. = 3.01-3.09; Color, white, gray, or pale blue, green, and brown ; Streak, white ; Luster, vitreous ; Translucent to opaque; a = 1.579; p = 1.593; -y = 1.597; -y- a = .018; Optically (-). B.B. Fuses easily at 2 with intumescence and yields a lithium flame, especially when fused with the fluorite flux. When fused with soda and dissolved in nitric acid, shows phosphoric acid with ammonium molybdate. Usually contains some water from the replacement of F by OH. General description. Crystals are coarse and not well formed, usually in cleavable masses. It occurs in the coarse granites and COLUMBATES, PHOSPHATES, VANADATES 513 pegmatites of Maine, as at Hebron, Paris, and Auburn, where it is associated with spodumene, lepidolite, and tourmaline ; also in North Carolina, and at Pala, San Diego County, California, with the same associated minerals. Chemically some sodium may replace the lithium, when the flame will be mixed with yellow. There has been a variety de- scribed in which sodium occurs alone, without lithium, forming a sodium amblygonite. OLIVENITE Olivenite. Cu 2 (OH)As0 4 ; Basic copper arsenate ; CuO = 56.1, As 2 O 5 = 40.7, H 2 = 3.2; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = .9396 : 1 : .6726 ; 100 A 110 = 43 13' ; 001 A 101 = 35 36' ; 001 A Oil = 33 55' ; Common forms, a (100), b (010), m (110), e (Oil), v (101) ; Cleavage, m, b, and e in traces ; Brittle ; Fracture, uneven ; H. = 3 ; G. = 4.1 ; Color, shades of dark green and brown ; Streak, green or brown ; Luster, vitreous ; Translucent to opaque. B.B. * Fuses easily, yielding a bluish green flame. On coal yields an arsenical odor, and after roasting and reducing with soda, borax, and coal dust yields metallic copper ; in the closed tube yields water. Soluble in nitric acid. General description. Crystals small, acicular, prismatic, or fibrous aggregates with a velvety surface. The brown varieties are known as " wood copper." Olivenite is a secondary mineral deposited in veins or cavities, associated with quartz in the oxidized zone of copper mines. Found in the United States in the Tintic district of Utah. LIBETHENITE Libethenite. Cu 2 (OH)PO 4 is the phosphate of copper isomor- phous with olivenite, and the two are therefore often crystallized together. It differs from olivenite in that the cold nitric acid solution yields a yellow precipitate with ammonium molybdate, (PA). Adamite, Zn 2 (OH)As0 4 , is the zinc member of the series; it is associated with libethenite in the old zinc mines of Laurium in Greece. 2L 514 MINERALOGY DESCLOIZITE Descloizite. ZnPb(OH)VO 4 ; A basic vanadate of lead and zinc; PbO = 55.4, ZnO = 19.7, V 2 O 5 = 22.7, H 2 = 2.2 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = 0.6368 : 1:0.8045; 100 A lib = 32 29' 40" ; 001 A 101 = 51 38'; 001*011 = 38 49' ; Common forms, a (100), m (110), b (010), o (111), f (201) ; Cleavage, none ; Brittle ; Fracture, uneven ; H. = 3.5 ; G. = 5.9 - 6.2 ; Color, orange-red, cherry-red, also shades of brown to black ; Streak, orange to brownish ; Luster, greasy ; Transparent to opaque. B.B. Fuses easily ; when reduced with soda, etc., on coal yields malleable lead buttons or a lead coat. Yields a green bead with the fluxes in R. F. and water in the closed tube. Easily soluble in cold dilute nitric acid which yields tests for vanadium, page 576. General description. Crystals are small prisms or pyramids, forming drusy surfaces on crusts ; more often amorphous, powdery or earthy. The lead and zinc may be replaced by manganese or iron and at times some of the V 2 O 5 is replaced by As 2 8 ; several such compounds have received separate names. Descloizite is found at Tombstone and various other localities in Arizona and New Mexico ; also at Leadville, Colorado ; and small quantities have been taken from the Wheatley mine at Phoenix- ville, Pennsylvania. Vanadium minerals are at the present time very valuable, as the vanadium is used in the manufacture of vanadium steel. A very small quantity of vanadium added to steel increases the toughness and the elastic limit without decreasing its ductility. Nickel accomplishes the same result, but vanadium is nearly twenty times as effective. CARNOTITE Carnotite. KUO 2 VO 4 . 1J H 2 O ; a potassium uranyl vanadate ; K 2 O = 10.37, U0 2 = 63.54, V 2 5 = 20.12, H 2 O = 5.95. A light canary-yellow mineral, disseminated as a yellow powder through sandstones in Montrose, San Miguel, and Mesa counties, Colorado, and the adjacent counties of Utah. It is easily soluble in acids and yields reactions for uranium and vanadium. It is a valuable mineral, not only for the large percentages of uranium and vanadium it contains, but also for the radium, which is associated with the uranium. COLUMBATES, PHOSPHATES, VANADATES 515 DUFRENITE Dufrenite. Fe 2 (OH) 3 P0 4 ; a basic orthophosphate of iron ; = 62.0, P 2 O 5 = 27.5 ; H 2 = 10.5 ; Orthorhombic ; Type, ? a:b:c = .8734:1:. 4262; 100 A 110 = 41 8'; Oil A Oil =46 10'; 001_A.101 = 26 I'] 001 A Oil = 23 5'; Common forms a (100), b(010), m(110), e(011); Cleavage, macropinacoidal distinct ; Brittle ; Fracture, uneven ; H. = 3.5-4 ; G. = 3.2-3.4 ; Color, dark green to nearly black, brown and yellow by oxidation ; Streak, green ; Luster, dull to silky ; Subtranslucent to opaque. B.B. Fuses in R. F. and becomes magnetic. Powdered and treated with H 2 SO 4 yields a green flame in O. F., or dissolved in nitric acid shows phosphoric acid with ammonium molybdate. Yields water in the closed tube and only iron reactions with the fluxes. General description. Crystals are small and cubical in appear- ance, but very rare, generally occurring as radiated or fibrous masses with drusy surfaces. Dufrenite is a secondary mineral, precipitated from solutions and associated with limonite deposits, as at Rockbridge County, Virginia. It is also found as a crust in the green sand formations of Allentown, New Jersey. By oxidation it becomes brown and by loss of P 2 6 forms limonite. LAZULITE Lazulite. (Fe . Mg) A1 2 (OH) 2 (PO 4 ) 2 ; when Fe : Mg: : 1 : 2, FeO = 7.7, MgO = 8.5, A1 2 O 3 = 32.6, P 2 O 6 = 45.4, H 2 = 5.8 ; Monoclinic ; Type, Digonal Equatorial ; a : b : c = .9749 : 1 : 1.6483 ; p = 89 13' = 001 A 100 ; 100 A 110 = 44 16' ; 001 A 101 = 58 49'; 001 A Oil = 58 45'; Common forms, P(lll), e (111), t (101) ; Twinning plane, 100 ; Cleavage, prismatic distinct ; Brit- tle; Fracture, uneven ; H. = 5-6; G. = 3.06-3.12; Color, azure- blue; Streak, white; Subtranslucent to opaque; a = 1.603; P = 1.632; y = 1-639; -y - a = .036; Optically (-); Axial plane parallel to 010; Bx a AC = 9 20'-9 45' behind; 2*E = 132 ; Pleochroism strong. B.B. Swells, whitens, and crumbles. The powdered mineral moistened with H 2 SO 4 yields a green flame (P 2 O 5 ) or fused with 516 MINERALOGY soda and dissolved in HNO 3 yields a yellow precipitate with ammo- nium molybdate (P 2 8 ). Becomes blue when treated with cobalt solution (Al). Insoluble in acids. General description. Crystals are plus and minus unit pyra- mids, or combinations of these with the unit orthodome ; other forms are rare. More often massive or granular, and associated with quartz and cyanite in slates. At Crowder Mountain, Gaston County, North Carolina, it is associated with corundum ; at Graves Mountain, Georgia, it occurs in fine sky-blue crystals, an inch or more in length, associated with rutile and cyanite. Crystals six inches long occur in pockets of quartzite in Wermland, Sweden. VIVIANITE Vivianite. Fe 3 (PO 4 ) 2 . 8 H 2 O ; Hydrous ferrous phosphate ; FeO = 43.0, P 2 5 = 28.3, H 2 - 28.7 ; Monoclinic ; Type, Digonal Equatorial ; a : b : c = .7498 : 1 : .7016 ; p = 75 34' = 001 A 100; 100 A 110 = 35 59'; 001 A 101 =49 46'; 001*011=34 11' ; Common forms, a (100), b (010), m (111), n (101) ; Cleavage, clinopinacoidal perfect, almost micaceous, very thin laminae slightly flexible and sectile ; H. = 1.5-2; G. = 1.58-2.68; Color, blue to green; Streak, white to blue, darkens on exposure; Luster, vitreous to pearly ; Transparent to opaque. FIG. 517. Vivianite. Leadville, Colorado. The smaller specimen is from Red Bank, N.J. COLUMBATES, PHOSPHATES, VANADATES 517 B.B. Fuses easily at 1.5, and yields a green flame (P20 5 ). In R. F. blackens and becomes magnetic. Yields water in the closed tube. Dissolves in acids, General description. Crystals are prismatic and flattened parallel to the orthopinacoid, which is often rounded and striated lengthwise; also in interpenetrating stellate groups, or radiated, encrusted, friable, and earthy ; at times replacing shells and roots and fossil bones, and is then known as bone turquoise. Vivianite is a secondary mineral formed by the action of solu- tions containing ferrous iron on apatite or other calcium phosphates of organic origin. It occurs in the clays and gravel beds as blue nodules at several localities in Monmouth County, New Jersey. It is also associated with limonite, as at Stafford County, Virginia ; replacing roots in clay at Eddyville, Kentucky. Groups of large crystals are obtained at Leadville, Colorado. ERYTHRITE Erythrite. Red Cobalt ; Cobalt bloom ; Hydrous arsenate of cobalt, Co 3 (A0 4 ) 2 . 8 H 2 ; CoO = 37.5, As 2 O 5 = 38.4, H 2 O = 24.1; Monoclinic; Type, Digonal Equatorial ; a : b : c = .7937 : 1 : .7356 ; p = 74 51' ; Common forms, a (100), b (010), m (110) ; Cleavage, b perfect, a and w (101) distinct, very thin laminae flex- ible and sectile ; H. = 1.5-2.5; G. =2.95; Color, crimson or shades of red and pink, sometimes grayish ; Streak, pale ; Luster, pearly on cleavage faces ; Transparent to translucent. B.B. Fuses on coal to a dark globule and yields an arsenic odor. In the closed tube darkens and yields water. With borax yields a blue bead (Co). Dissolves in HC1 to a rose-colored solu- tion. General description. Crystals prismatic, in stellate or radiated aggregates; in drusy crusts or earthy. Associated with cobalt ores as an oxidation product. Beautiful radiated specimens are obtained at Schneeberg in the Harz and at Freiberg in Saxony. It is found as drusy crusts and as an eartHy powder associated with the cobalt ores of Cobalt, Ontario. The corresponding nickel mineral, Annabergite, Ni 3 (As0 4 ) 2 . 8H 2 O, is apple green and is also found associated with the ores of cobalt and nickel as an oxidation product. 518 MINERALOGY WAVELLITE Wavellite. A1 3 (OH)3(PO 4 )2 . 5 H 2 O ; hydrous basic aluminium orthophosphate; A1 2 3 = 38.0, P 2 O 5 = 35.2, H 2 = 26.8; Or- thorhombic ; Type, Didigonal Equatorial a : b : c = .5573 : 1 : .4084 ; 100 A 110 = 26 47' ; 001 A 101 = 36 36' ; 001 A Oil = 20 33'; Common forms, m (110), b (010), p (101) ; Cleavage, p and b quite perfect; Brittle; Fracture, uneven; H. = 3.25^; G. = 2.3-2.33; Color, shades of green, yellow, white to gray, when impure brown to black; Streak, white; Translucent to opaque; Luster, vitreous to pearly ; -y a = .025 ; Optically ( + ) ; Axial plane = 100; Bx a = c; 2E = 127 2'. B.B. Infusible, but whitens and crumbles somewhat ; becomes blue with cobalt solution. The nitric acid solution yields a yellow precipitate with ammonium molybdate (P 2 5 ). Soluble in hot strong acids. General description. Separate crystals are very rare ; usually occurs in concretionary masses with drusy surfaces and radiated structure ; also stalactitic or in crusts. Small amounts of iron or manganese may replace the aluminium, and while it is not recog- nized in the formula, fluorine is nearly always present. Evansite, A1 3 (OH) 6 P0 4 . 6 H 2 0, peganite, A1 2 (OH) 3 PO 4 . 1| H 2 0, and sphaerite, A1 5 (OH) 9 (P0 4 ) 2 . 3^ H 2 O, are other phosphates of aluminium with varying amounts of water, very closely related to wavellite. They are all secondary minerals produced by the interaction of percolating waters, containing phosphates in solution, with argil- laceous shales and slates, in the cracks and cavities of which they occur, never forming deposits of great extent. Wavellite is associated with limonite at White Horse Station, Pennsylvania; at Magnet Cove, Arkansas, in fine green radiated aggregates. TURQUOISE Turquoise. CuO . 3 A1 2 3 . 2 P 2 5 . 9 H 2 ; CuO = 9, A1 2 = 36.50, P 2 O 5 = 34.13, H 2 O = 20.12; Triclinic a : b : c = .791: 1: .605; a = 92 58', P = 9_3 30', y = 107 41'; 010 A 100 = 44 50'; 100 A 110 = 31 10; 011*110 = 105 36'; Forms b (010), a (100), m(110), M(110), k(011); Cleavage, marked; Brittle; Fracture, COLUMBATES, PHOSPHATES, VANADATES 519 conchoidal ; H. = 6 ; G. = 2.6-2.86 ; Color, sky-blue to apple- green ; Streak, white to pale green ; Luster, waxy ; Subtranslucent to opaque; a = 1.61, v = 1.65, y a, = .04. B.B. Infusible, becoming brown and glassy, yields a green flame. The S. Ph. bead reduced with tin shows copper. In the closed tube yields water. General description. Crystals of Turquoise were discovered for the first time at Lynch Station, Campbell County, Virginia, and de- scribed within the year. They were very small and scarce. Usually occurs in amorphous masses filling small veins in altered porphy- ries. In the United States turquoise is mined extensively at Mineral Park, Mohave County, Arizona; Washoe County, Ne- vada ; Burro Mountains, New Mexico. The blue color is due to copper, and in many specimens fades on exposure. The finest gem turquoise is found near Mishapur, Persia. Turquoise has been used as a gem from remote ages, as the bracelets discovered at El Mehesna, the oldest known jewels, contained beads of turquoise alternated with beads of gold. The American material is slightly soft and porous, which affects the polish. Variscite. A1P0 4 . 2 H 2 O ; a crystalline hydrous phosphate of aluminium, very similar to turquoise in color, but lighter green to deep emerald green. It contains no copper and occurs in south- western Utah, where it is found as nodules contained in a lime- stone, associated with jade, chalcedony, and limonite. TORBERNITE Torbernite. Hydrous uranyl phosphate of copper, Cu(U0 2 ) 2 - (PO 4 ) 2 . 8 H 2 0; Cu = 8.4, U0 3 = 61.2, P 2 5 = 15.1, H 2 O = 15.3; Tetragonal; 6 = 2.9361; 001*101=71 11'; Forms, c(001), m(110), a (100), e (101) ; Cleavage, basal micaceous; Brittle; H. = 2-2.5; G. = 3.4-3.6; Color, emerald or siskin-green ; Streak, pale green ; Luster, pearly ; Transparent to opaque. B.B. Fuses at 2.5 to a black slag and yields water in the closed tube. Reduced with soda, etc., on coal, yields copper buttons. The nitric acid solution shows phosphoric acid with ammonium molybdate, also yields tests for uranium, page 576. 520 MINERALOGY General description. Crystals are thin plates or tabular; also in foliated and micaceous aggregates. Chemically some arsenic may replace the phosphoric acid. Zeunerite, Cu(UO 2 )2(As0 4 ) 2 . 8 H 2 O, is the arsenic mineral isomorphous with torbernite and very simi- lar to it, except in color, which is lemon or sulphur yellow. Autunite, Ca(U02) 2 (PO 4 ) 2 . 8 H 2 O, is a member of the same group, but orthorhombic and lemon or sulphur yellow in color. All three minerals are secondary oxidation products associated with uranium deposits of Joachimsthal, Bohemia. Torbernite is associated in small amounts with the carnotite at Richardson, Utah. NITRATES SODA NITER Soda Niter. Chili Saltpeter, NaNO 3 ; Nitrate of soda ; Na 2 O = 36.5, N 2 O 5 = 63.5; Hexagonal; Type, Dihexagonal Alternat- ing; c = .8276; 0001 A 1011 = 43 42'; r*r'= 73 30'; Cleavage, rhombohedral perfect ; Fracture, conchoidal ; Brittle ; H. = 1.5-2 ; G. = 2.24-2.29 ; Color, white, gray, red, brown, or yellow ; Trans- parent; CD = 1.587; = 1.336; CD - = .251; Optically (-). B.B. Deflagrates on coal, colors the flame intensely yellow (Na). Has a cooling taste, easily soluble in water and yields reac- tions for nitrogen, page 590. General description. Crystals are rare, usually in beds, crusts, or granular. It is isomorphous with calcite, though differing from it entirely chemically. All nitrates are very soluble in water, and their occurrence in nature is therefore restricted to arid regions or to caves where little water has access. Nitrates are formed in the soils by the oxidation of organic matter through the action of certain bacteria. Nitrates are carried in the ground water and serve as a supply for growing vegetation. Nitrogen is one of the most expensive as well as the most important plant foods, and for this reason the nitrate deposits of Chili are of enormous commercial importance, as they are the only extensive deposits of nitrogen salts in the world. The origin of these beds has as yet not been satisfactorily explained. They may have been deposited by evaporating solutions, by volcanic action, or by decaying organic materials. They extend over an area of many square miles in COLUMBATES, PHOSPHATES, VANADATES 521 northern Chili, southern Peru, and Bolivia. The nitrate is asso- ciated with salt, gypsum, glauber salts, and generally borax. Small amounts occur in Humboldt County, Nevada, and in San Bernardino County, California. NITER Niter. Saltpeter, Potassium Nitrate, KNO 3 ; K 2 = 46.5, N 2 6 = 53.5 ; Orthorhombic ; Type, Didigonal Equatorial ; & : B : c = .5843 : 1 : .7028 ; 100 A 110 = 39 35' ; 001 A 101 = 49 30' ; 001 A Oil = 35 2'; Common forms, b (010), m(110), t (021), o(lll), q(011); Cleavage, Oil perfect, 010 and 110 imperfect; Brittle; Fracture, uneven; H. = 2; G. = 2.09-2.14; Color and streak, white; Luster, vitreous; Subtranslucent ; a = 1.334; p = 1.505; 7 = 1.506; "y - a = .172; Optically (-); Axial plane = 100; Bx a = b; 2E = 8 40'. B.B. Deflagrates on coal, yielding a violet flame (K), has a cooling saline taste. Easily soluble in water, the solution yields reactions for nitrogen, page 590. General description. Crystals are acicular, forming crusts or silky tufts, as efflorescent crusts in dry regions. It is an oxidation product found in soils, the result of the action of certain nitrifying bacteria; the nitric acid thus formed combines with the bases, with potassium to form niter, or with calcium to form nitrocal- cite, Ca(NO 3 ) 2 . n (H 2 0). Niter is dimorphous, and in each form is isomorphous with the two forms of calcium carbonate, calcite and aragonite. The or- thorhombic form here, however, is stable at ordinary temperatures. The hexagonal phase of soda-niter is stable at ordinary tempera- tures and forms the natural occurring salt. Niter is of great commercial importance both as a fertilizer and in the manufacture of gunpowder, the natural supply being so limited that the salt is formed from Chili saltpeter by the interac- tion of potassium chloride. Niter is associated to some extent with the sodium salts in the Chili nitrate beds. It also occurs as an impregnation in the earth on the floors of some of the caves in Kentucky and Tennessee. It is often obtained by lixiviating such soils. 522 MINERALOGY BORACITE Boracite. Mg 7 Cl 2 Bi 6 03o ; A chloride and borate of magnesium ; MgO = 31.4, Cl = 7.9, B 2 O 3 = 62.5; Pseudo-isometric; Type, Ditesseral Polar; Common forms, a (100), d(110), o(lll); Twinning plane, 111; Cleavage, 111 traces; Fracture, conchoid al ; Brittle ; H. = 7 ; G. = 2.9-3 ; Color, white, gray, pale yellow, or green; Streak, white; Transparent to translucent; Double re- fraction below 265 ; a = 1.662; p = 1.667; (lll); Cleavage, a distinct, and b less so; H. = 2.5-3; G. = 2.06-2.2; Color, white to dark flesh-red or yellow; Streak, white; Luster, vitreous; Transparent to trans- lucent; Optically (-); Axial plane, 010; Bx a A c = 10 45' in front; 2V = 84 33'. B - B - ~- Yields a potassium flame. Soluble in water, the solu- tion yields a white precipitate with barium nitrate (BaS0 4 ), filtered and acidified with nitric acid yields a white precipitate with silver nitrate (Cl), or reacts for chlorine with copper oxide. Will not effervesce with acids. Has a bitter, saline taste. General description. Crystals are combinations of the plus and minus unit pyramids and prism, with the pinacoids at times. SULPHATES, CHROMATES, ETC. 535 The base is usually rough and uneven. It is more often granular or massive. Kainite being very soluble in water has been deposited from con- centrated sea water ; when this concentration has reached the stage where the sulphates have been deposited and the mother liquor is saturated in respect to the chlorides and sulphates, double salts are separated, of which kainite is an example. This mineral, however, may have been formed by the interaction of car- nallite (KC1 . MgCl 2 ) .6 H 2 O and kieserite (MgSO 4 ) . H 2 O as a secondary mineral. Kainite is found in quantities at the unique salt deposits of Stassfurt and in small deposits of the same character in Galicia. These two deposits are of great commercial value, as they furnish the potash supply to the world, and which is essential as one of the necessary plant foods, so apt to be early exhausted from the soil. HANKSITE Hanksite. 4 Na 2 SO 4 . Na 2 C03. A double salt found under the same conditions as kainite, and interesting as one of the few miner- als illustrating the dihexagonal equatorial type. Its crystals are tabular combinations of the base, unit pyramid, and prism. It is found associated with the borax lake deposits of California. MIRABILITE Mirabilite. Hydrous sodium sulphate, Na 2 S04 . 10 H 2 0; Na 2 O = 19.3, SO 3 = 24.8, H 2 O = 55.9; Monoclinic; Type, Digonal Equatorial ; a:b:c = 1.1158: 1: 1.2372 ;~~p = 72 15' = 001 A 100 ; 100 A 110 - 46 44' ; 001 A 101 = 57 55' ; 001 A Oil = 49 41' ; Common form, a (100), b (010), c (001), m (110) ; Cleavage, a perfect, c and b in traces; H. = 1.5-2; G. = 1.48; Color and streak, white ; Luster, vitreous ; Transparent to opaque ; p = 1.44 ; Optically (-); Axial plane J_ 010; Bx a = b ; 2E = 122 48'. B.B. Boils and yields a yellow flame (Na). In the closed tube yields much water. After ignition leaves an alkaline residue. Very soluble in water, the solution yields a white precipitate with barium chloride (BaSO 4 ). Has a cooling, bitter taste. General description. Occurs as crusts or in beds in the de- posits formed by the evaporation of salt lakes. Sodium sulphate is contained in varying amounts in all natural waters ; upon con- 536 MINERALOGY centration mirabilite separates from the saturated solution when the temperature is below 32 ; when the temperature is higher, the anhydrous salt, thenardite, is separated. ' Its solubility varies greatly with the temperature, thus at Great Salt Lake, Utah, on cold days in winter large amounts of sodium sulphate are thrown up on the shore by the waves, only to be redissolved when the temperature rises. In a dry atmosphere it loses its water of crystallization, falling down as a fine white powder. It also occurs as an efflorescence on rocks or near springs, where much water is quietly evaporating. GYPSUM Gypsum. CaSdi . 2 H 2 O ; Hydrous calcium sulphate ; CaO = 32.5, SO 3 = 46.6, H 2 = 20.9; Monoclinic; Type, Digona! Equatorial; a:b: c= .6899 : 1 : .4124; p = 80 42' = 001 A 100; 100,110 = 34 15'; 001*101 = 28 17'; 001 A Oil -22 9'; 111*111 =36 12'; Common forms, b(010), c (001), m (110), l(lll), n(lll); Twinning plane, 100, contact and crossed pene- trating, also 101 less common ; Cleavage, clinopinacoidal perfect, a less so, and 111 fibrous; laminae flexible parallel to the fibrous cleavage; H. = 1.5-2; G. = 2.3-2.33, when pure; Color, white, pale yellow, red, brown to black when organic matter is present; Streak, white ; Luster, silky, pearly, vitreous to dull ; Transparent to opaque; a = 1.5204; (* = 1.5229; -y = 1.5296; y - a = .009 ; Optically ( + ) ; Axial plane = 010 ; Bx a A c = 52 30' in front; 2V = 58 8'. B.B. Whitens and fuses to an opaque white mass at 3 ; colors the flame yellowish red ; after ignition reacts alkaline with turmeric paper. In the closed tube yields water. Fused with soda and a little coal dust in the R. F. reacts for sulphur with silver. Soluble in HC1. General description. Crystals usually simple combinations of the plus and minus unit pyramids with the unit prism and the clino- and basal pinacoids. Crystals six feet in length have been found in Wayne County, Utah. The faces m and b often striated parallel to their intersection with the base. Parallel growths and rounded stellate aggregates are common, as at St. Mary's River, Maryland, and Postelberg, Bohemia. Twinning in which the composition plane is parallel to the orthopinacoid, forming the well-known swallow-tail twins, and, when the crystals are rounded, SULPHATES, CHROMATES, ETC. 537 the arrowhead twins, is common at Montmartre near Paris. Simple crystals are more often found in clays, as at Poland, Ohio. All crystalline gypsum which shows the perfect cleavage is known as selenite. The fibrous variety with a satiny or pearly luster and a fibrous fracture is satin spar, while the granular massive variety is alabaster. Rock gypsum is an impure granular form, often earthy. Gypsum is deposited from solution and is associated with sedimentary rocks, limestones, and clays, from which the soluble calcium sulphate has been leached out. It is also associated with salt deposits, being deposited from the concentrated brines before FIG. 521. Gypsum. Poland, Ohio. the more soluble sodium or magnesium sulphates and chlorides, and therefore in the usual position underlying the salt, or is near it in position ; at times, when there have been several distinct periods of concentration, it may be interbedded with salt and shales. Large beds of rock gypsum are found in the Salina formations of New York, but here the gypsum beds are above the salt and are probably independent of it, the concentration of the solution hav- ing been interrupted before the salt was deposited. Large beds of gypsum are found in Nova Scotia, Newfoundland, Michigan, and in the borax lakes regions of California and Nevada. Gypsum is also formed near volcanoes and fumaroles ; small crys- tals of gypsum cover the walls of the lava caves of Kilauea. Commercially rock gypsum is ground and used as a fertilizer. The purer varieties, when heated at a temperature below 130 C., 538 MINERALOGY until one molecule of the water of crystallization is driven off, form a cement known as " plaster of Paris," named from the Mont- martre deposits near Paris where this cement was first made. This calcined product when moistened absorbs water, forming a network of fibrous crystals, and solidifies as a whole. When all the water of crystallization is driven off, it forms anhydrite; the product loses its power to absorb water or absorbs it very slowly, and its setting or crystallizing power is lost. Satin spar and alabaster are polished as ornamental stones and for inexpensive jewelry. EPSOMITE Epsomite. Epsom salts, MgS0 4 . 7 H 2 ; MgO = 16.3, S0 3 = 32.5, H 2 O = 51.2; Orthorhombic ; Type, Digonal Holoaxial ; ft : b': c = .9902:1:.5709; 100 A 110 = 44 43'; 001*101 = 29 58' ; 001 A Oil = 29 43' ; 111 A 111 = 52 38' ; Common forms, a (100), b (010), c (001), z (111), n (101) ; Cleavage, b perfect, Oil less so ; Brittle ; Fracture, conchoidal ; H. = 2-2.5 ; G. = 1.68-1.75; Color and streak, white; Transparent to trans- lucent; a = 1.432; p = 1.455; y = 1.461; . y - a = .029; Optically (-); Axial plane = 001; Bx a = b; 2E = 78 20'. B.B. Boils and yields an infusible white alkaline residue which becomes flesh-pink when treated with cobalt solution (Mg) . Soluble in water ; the solution yields a white precipitate with barium chloride (BaSO 4 ). It has a very bitter taste. General description. Crystals are prismatic in habit, combi- nations of the sphenoids and unit prism; they are interesting as examples of the holoaxial type ; also in fine silky acicular crys- tals. The mineral is named from the locality of Epsom Springs, England, where it was first known. Magnesium sulphate is easily soluble in water ; all springs and percolating ground waters contain both magnesium and calcium sulphates in considerable quantity. They cause the permanent hardness of natural waters. Where large amounts of water are evaporating, as on the face of cliffs or on the surface of the soil in very dry seasons, epsomite is left as white crusts. The white crusts formed on fresh brick walls are in part epsomite. Such residual crusts occur on the floors of the caves of Tennessee and Kentucky and on many of the alkaline plains of California, Utah, and Nevada. SULPHATES, CHROMATES, ETC. 539 Kieserite, MgSO 4 . H 2 O, is a magnesium sulphate containing only one molecule of water. It is monoclinic and much less soluble than epsomite, but dissolves slowly in water and recrystallizes as epsomite at ordinary temperatures. Kieserite separates from solu- tions above 68 C. Kieserite is associated with carnallite and gyp- sum at the Stassfurt salt deposits. Here it has been separated from solutions containing sodium and potassium salts, and under these conditions a number of double salts have been formed, as blodite, Na 2 Mg(SO 4 ) 2 4 H 2 O ; loweite, Na 2 Mg(S0 4 ) 2 . 2J H 2 O ; picromerite, K 2 Mg(SO 4 ) 2 .6H 2 O. Isomorphous with epsomite is the zinc sulphate, goslarite, ZnS0 4 . 7 H 2 0, derived from the oxidation of sphalerite and occurring on the walls of old mine workings; also morenosite, NiSO 4 . 7 H 2 O ; the nickel sulphate is a member of the same group. Commercially magnesium sulphate is separated at the Stassfurt works. It is used in medicine as a purgative, and as a coating for cotton cloth in dyeing. MELANTERITE Melanterite. Copperas ; Ferrous sulphate, FeS0 4 . 7 H 2 O ; FeO = 25.9, SO 3 = 28.8, H 2 O = 45.3; Monoclinic; Type, Digonal Equatorial; a: b: c = 1.1828 : 1 : 1.5427; p = 75 44' = 001 A 100; 100,110 = 48 54'; 001 A 101 = 43 44'; 001 A Oil = 56 13'; 111 A 111 = 78 33'; 001 A 110 = 80 41'; Common forms, b (010), c (001), m (110), r (111), o(011), v(101); Cleavage, basal perfect, m less so; Brittle; Fracture, conchoidal; H. = 2; G. = 1.89-1.90; Color, shades of green to yellow; Streak, white; subtransparent to translucent ; a = 1.471; p = 1.478; y = 1.485; = 1.572; = 1.592; o> = .020; Optically (+). B.B. Infusible, but may decrepitate when ignited ; treated with cobalt solution becomes blue (Al). In the closed tube yields water, and fused in R. F. with soda and a little coal dust yields a sulphur reaction on silver. Insoluble in HC1, soluble in H 2 SO 4 . General description. Crystals are small and rhombohedral in habit, usually combinations of several rhombohedrons of the same series. More often massive, granular, or of a fibrous-like structure. Alunite is very local in its occurrence, and it has been produced by the action of sulphurous fumes on the feldspars of such rocks as rhyolites, andesites, or trachytes, or by the decomposition of these rocks by percolating waters containing sulphuric acid, as in the Goldfield district of Nevada ; here the formation of alunite has a direct connection with the workable deposits of gold. It also 542 MINERALOGY occurs at Cripple Creek, Colorado; in Mariposa County, Cali- fornia ; near Morenci, Arizona. Alunogen, A1 2 (SO 4 ) 3 . 18 H 2 0, hydrous sulphate of aluminium, is soluble in water and occurs as an efflorescence on the walls of coal mines. Formed by the action of sulphuric acid on shales. A large deposit, fibrous in character, occurs at Smoky Mountain, North Carolina. Aluminite (A10) 2 SO 4 . 9 H 2 0, a basic sulphate insoluble in water, is found in concretionary forms imbedded in clay. Kalinite, KA1(S0 4 ) 2 . 12H 2 O, is a natural potash alum, found as an efflorescence on slates and on the walls of caves of Tennessee. All these minerals where found in sufficient quantities are used in the manufacture of soluble aluminium salts and alum. WOLFRAMITE Wolframite. Tungstate of iron and manganese, (Fe . Mn) W0 4 ; when Fe : Mn: : 4 : 1, FeO = 18.9, MnO = 4.7, W0 3 = 76.4 ; Monoclinic ; Type, Digonal Equatorial ; a : b : c = .8300 : 1 : .8678 ; p = 89 21' = 001 A 100 ; 100 A 110 = 39 s 41' ; 001 A 101 = 45 56'; 001.011 =40 57'; Common forms, a (100), m(110), t (102), y (102), o (111) ; Twinning axis c, composition plane, 100; Cleavage, b perfect ; Brittle ; Fracture, uneven ; H. = 5-5.5 ; G. = 7.2-7.5 ; Color, dark brown to nearly black ; Streak, brown- ish or reddish to nearly black; Luster, metallic adamantine to dull ; Opaque, rarely translucent. B.B. Fuses at three or four to a globule which in R. F. is usually magnetic. When dissolved in the S. Ph. bead and reduced with tin on coal yields a blue solution when dissolved in HC1. Fused with soda in 0. F. yields green sodium manganate. General Description. Crystals tabular with 100 prominent, or stout prismatic, striated on 100 parallel to the vertical axis. Also in granular masses. Wolframite is an isomorphous mixture of hiibnerite, MnW0 4 , the tungstate of manganese, and ferberite, FeWO 4 , the tungstate of iron. Hiibnerite occurs as brown, translucent, bladed crystals,' while ferberite is black and opaque. The three minerals occur under the same conditions, usually in quartz veins in granites, associated with sulphides ; here they have probably been precipitated from hot solutions. They also occur SULPHATES, CHROMATES, ETC. 543 in pegmatites, associated with cassiterite, as in Cornwall, England ; in the Black Hills, South Dakota ; also in the Seward Peninsula, Alaska. In all such cases their origin, like that of the cassiterite, is due to pneumatolitic agencies. A third but less common occur- rence is the replacement in limestone, as at Turnbull, Connecticut. Wolframite and hubnerite occur in numerous localities in the Western states, associated with gold-bearing quartz veins, but al- ways in small amounts. Wolframite is mined on a commercial scale in Boulder County, Colo- rado. Hubnerite was first de- scribed or obtained from the Enterprise mine, Nevada, where it is associated in a vein with apatite, fluorite, and scheelite. Commercially wolframite is the principal source of the metal tungsten and its salts. The metal is added to steel in the form of ferrotungsten, produc- ing a tungsten steel which will retain its temper when working at or near a red heat ; from this steel, lathe tools, drills, hack saws, etc., are manufactured. Incandescent lamp filaments made of tungsten yield a very white light and reduce the cur- rent used to 1J watts per candle power, while the carbon fila- ment requires three watts. Sodium tungstate is used in fireproof- ing curtains and draperies ; as a mordant in dyeing. Calcium tung- state is the phosphorescent salt with which the screen used in viewing the Rontgen rays is coated. Artificial wolframite may be produced by fusing sodium tung- state and the chlorides of sodium, manganese, and iron. When the iron is left out, hubnerite is the result. FIG. 522. Wolframite Crystal from Zinnwald, Bohemia. SCHEELITE Scheelite. Calcium tungstate, CaWO 4 ; CaO = 19.4, W0 3 = 80.6; Tetragonal; Type, Tetragonal Equatorial ; c = 1.5356; 544 MINERALOGY 001 A 101 =56 55'; 111 * 111 = 79 55'; 101 A Oil = 72 40'; Com- mon forms, p(lll), e(101), c(001), h (313), s (311) ; Twinning plane, 100, both contact and interpenetrating; Cleavage, 111 distinct, e interrupted ; Brittle ; Fracture, uneven ; H. = 4.5-5 ; G. = 5.9-6.1 ; Color, white, pale yellowish white, pale yellow to brown; Streak, white; Luster, adamantine to vitreous; Trans- parent to opaque; (0 = 1.934; = 1.918; O CO CD OO 00 CO CO iO iO O5 OiC^IO(NOT^ "o S 2.2 s .g S S O .1 ^^ ^a Q CO 00 CO I oo 1 . ^. o Tt< t> rH -III C2 Q o o o o o I- o to 5 '3 3 .lais a -a-i-ii o S 1 -i Tetrahe Cobaltite 5 ti S GO (N O5 to Oi (M CO (N CO CO CO 598 MINERALOGY THE MORE COMMON MINERALS 599 I O g fe * O ^ 2 E 3 P a d 5 S o rrt r3 d 05 rS s ft Q g ^ ft M OJ g I J g J-.S d 3 .1 X ^| O I l! 1 ^ 'g J ? o ll ti OJ jO fe 13 O >^ 4S ,0^2 CS^d rO Til L t M *L K*J rh -H F*5 ^ O 2 O o X5 ^ t O! 'd'd II t O co co 10 ^ lO ^ ^O ^? *O cq (N* co co ft\s o -r Q ^3 d *5 3 pt .S SI g^ CO co 600 MINERALOGY !? S cd ^ r^ h* 1 M **H H g g * ri S B &H 5 m EI ^ ^ Q S 2 e q W S Q COLOR O MINERA !-< ^ S ft ^ ft- .2 CO 00 CO O5 . . . . I I-H ,_5 T-4 T-4 (N (N (N iO O CO lOiOiO R ord'3 -22 o ?H -1-3 R k> . ^Q O c3 rS co E J R e3 .- o3 j.94 Illlll M-HI1 i l >< e3 > '-1 fcOQ ft a 5 -S .2 "eS d a O 03 I & r 1^5 qj rt .S^S 3^ II II M PQ I 1-1 co (N PQ J 10 O l>- T^ Oi O CD O5 O ^ Ttn CO ^ IO 602 MINERALOGY M "i a II a I c 05 CO M CO CO CO CO W d d d 000 > 3 ^ S* & . o QJD or ^ fl3 . "* O > >> o> e S - "^ fe Oi ?! g:^ > K*S O CO CO CO CO co ^ CO CO 3 3 I CO CO T T coco CO CO .15 'd j Q M fl 11-11 -S he ri S 8 * ^rS > _5 S3 J PH cc o: co *o 10 co ^o co r-H i^ O, jH CO 10 THE MORE COMMON MINERALS 603 I d &JO ! .2 3 . ft 2 grf a 1|S is! "3 , s ta ||S 111 !>. S O2 OQ VI mplanted XI O - OH ^ a ^ i ' ?H CS i I - a, massive rough, prismati XI SXX 4-3 I 03 , sa ped 11? a t^.2 c 5 -e ti o a (M (N O5 CO CO H M 13 3 t>i Q rious of fl I ^ I si 2 3^.2 ^ i JS 03 CD S 1 > -^ r ^If e 5' ^ Wh Wh various ite, pinki o & - s P -i- 3 > bxi Sa-rf's ? ^2" VI X S-.2-& .2 ill ||1 rsll ^ E ^3 2, ^ 8^ 13 PQ ^ iO 1C 1 up up up iC 1C ^C 1C iC iC iC 1C LC 1C 10 1 J ill 1 I 1 W fe O<1a2 <3 O i> Pectoli Smiths Analc Natro 00 t> 1-1 (M Oi ^ co >c 604 MINERALOGY B '| S? s X! H oT ^ K, OX) SGO " ' GO .JH "S S ^J *i > ^ JS - a II 1 00 C4 d i-rt i I CD II .1 J5 g s i *^ ^ CD CD H .2 3 d d '^ 6 i ii ;-' CD CD A ft CO -^ 2 CO lO CO l> CO CO CO CO rH CO CO THE MORE COMMON MINERALS 605 O 1 ^O 'o 02 X ve. bb ed nd s^ 2 I JU ft .- * a i 00 M Qj a s > -g| | g'ft a c^ "S s^r E 3 sq sto "o r-: O rg -4-3 OO -3 I '^' s ?.2 00 4^ o d S I OO aT fe .23 -2 .3 JIMI i "a -i i i 2 rf-1 a I ^3 > >< 2 m S = 8 a - T3 ^^ g ft ^^ K &'~ m " CP ^ a I 1" "g Is 13 cp e3 d S S 2 gJt' oa ^ J2 J2 ' XX onal pri \ 1 11 w oo <3 S - a ft ft t: a a a s - CO CO CO 606 MINERALOGY { I B o ^ :3^ "S *s "rrt 3 M 5-8 fibrous, 1 resinous, and fine, sm sm Xl Xl Xl ,j 53 ^ l-H ^ I.',l ''*' -S'8 S (5 | || 02 ri I -I *$1 2. "is eg 03 i-rt > ^H OD n "o a; d > ^J 03 g J s||a| ifll^ 8 !"!* 02 S < r^ 03 _M R " , Cleava Botryo 1 1 t 1 d hH t I O hH hH <] 1 o 10 CO 10 rH 4 1 CO (N CO o I jj^ll I 1 ! 1 !*!! s & & tf 2 rH rH ^ CO CO 10 10 ' J3 te nite 1 1 L w m N iJ n mo THE MORE COMMON MINERALS 607 |_l |_H HH I I fl &| a M ^ a a 03 OS . 00 CO CO CO Js ft ^ & O ^ -g a 1 49 +3 ^O *o t. .3 'o c 5 j ^. a o o flT d . t^ > .* d ^^ ^ **"* 42 W2 d CD M 1 3 O a SYSTEM > > 1 hH -4-=> 1 3 1 o 1 | 'o CD I i 43 H ft d 8 J O * I o O P GRAVITY 2.5-2.7 00 CO ti CO 5 O M o < I11S 1 CHAPTER III TABLE FOR THE DETERMINATION OF THE PRINCIPAL ROCK-FORMING MINERALS IN SECTIONS I. The mineral is opaque. Magnetite. Isometric. In crystalline outlines, rounded grains, or dustlike. By reflected light, bluish black in color ; p. 373. Chromite : Isometric. In crystalline outline, or rounded grains. By reflected light brownish and at times the very thin edges may appear brownish ; p. 376. Ilmenite. Hexagonal. Tabular, thin plates, or irregular in outline. By reflected light brownish and on very thin edges brown; p. 346. Hematite. Hexagonal. In thin plates, scales, and grains. The thin edges and scales dark red by transmitted light ; p. 343. Graphite. Hexagonal. In thin flakes, foliated masses, and grains. By reflected light, metallic luster or dull black ; p. 284. Pyrite. Isometric. Crystalline outlines, irregular masses, or rounded grains. By reflected light yellow, metallic luster; p. 313. Pyrrhotite. Hexagonal. Irregular grains and masses. By reflected light bronze-yellow with a metallic luster ; p. 308. II. Transmits light isotropically. A . Index of refraction below that of Canada balsam. Fluorite. Isometric: n= 1.43. In irregular masses or rounded grains, rarely showing crys- talline outlines, filling cavities or veins; Colorless, violet or purple; Cleavage cracks well developed; Relief marked, from the low refraction; p. 331. Opal: Amorphous: n= 1.44. 2 R 609 610' MINERALOGY In irregular masses, grains, and filling veins, often forming the cementing material in sandstones and shales; Colorless or nearly so; In sections may be mistaken for leucite or so- dalite; p. 369. Glass. Amorphous; n = 1.49; Variable. As the residuum of crystallization, irregular in outline, filling the spaces between the crystals ; Colorless, gray, or smoky. Analcite. Isometric; n = 1.48. Rounded crystalline outlines, cloudy grains, or filling cavities and 'veins ; Cleavage cubic but not well developed ; Color- less ; Usually a secondary mineral after leucite, sodalite, or nepheline ; p. 485. Sodalite group. Isometric; n = 1.48-1.50. In six- or eight-sided crystalline outlines or rounded grains ; Cleavage' not well developed, often irregularly cracked and fractured; forming the ground mass between crystals of other species ; Usually colorless but often yellow or blue, especially haiiynite and noselite ; Often filled with dustlike inclusions, arranged in zones or collected at the center or margin; p. 436. Leucite. Isometric; n = 1.50. In six- or eight-sided crystalline outlines, or rounded grains ; Cleavage not well developed, often irregularly fractured; inclusions when present are arranged symmetrically in zones or radiating from the center ; Colorless ; Relief very low; Large crystals are doubly refracting; Interference color first order gray, showing polysynthetic twinning; p. 415. B. Index of refraction above that of Canada balsam. Spinels. Isometric ; n = 1.71-1.76. In square or six-sided crystalline outlines or rounded grains ; Cleavage not developed ; Colorless to opaque, according to the species ; Relief very marked and surface very rough ; p. 371. Garnet. Isometric; n = 1.75-1.85. In crystalline outlines, rounded grains or irregular masses; Cleavage not developed ; Irregularly fractured and cracked ; Relief very marked and surface rough ; Colorless, to reddish of various shades, also green, brown, to nearly opaque ; p. 442. III. Transmits light anisotropically. A. Uniaxial. THE PRINCIPAL ROCK-FORMING MINERALS 611 1. Index of refraction less than that of Canada balsam. a. Double refraction equal to or less than that of quartz ; not pleochroic. 1 Nephelite. Hexagonal; n = 1.54; a) = .005. In nearly square or hexagonal outline, irregular masses, and rounded grains ; Colorless, gray, greenish, bluish or brown ; Cleavage prismatic and basal, though not well developed in sections ; Relief very flat ; Inter- ference color low first order gray ; Optically negative (-); P. 440. Tridymite; Hexagonal; n = 1.477; o) = .002. In hexagonal plates and scaly aggregates; Colorless and transparent ; The surface appears rough from the low refraction; Interference color very low gray of the first order ; Optically positive (+) ; p. 361. Quartz; Hexagonal;, n = 1.547; to = .009. Irregular grains and angular masses ; No cleavage or relief; Interference colors first order gray to yellow; Optically positive (+) ; p. 352. 2. Index of refraction greater than that of Canada balsam, a. Double refraction less than that of quartz. Apatite; Hexagonal; n = 1.635; co - = .004. Hexagonal sections, prismatic elongated sections, or rounded grains; Cleavage not developed, elongated sections show transverse parting; Colorless; Not pleochroic ; Relief not marked ; Interference colors first order gray ; Optically negative ( ) ; p. 508. Vesuvianite; Tetragonal; n = 1.72; (0 = .006. In short square prisms, grains, or irregular masses ; Cleav- age not developed ; Colorless, yellow, green, brown, or blue ; Relief marked, surface rough ; Slightly pleo- chroic ; Interference color first order gray, often anomalously high; Optically negative ( ) ; p. 455. Corundum; Hexagonal; n = 1.766; (0 - = .009. Crystalline outlines, elongated parallel to c, grains or irregular masses ; Rhombohedral parting ; Colorless, red, or blue ; Pleochroic only in highly colored sec- tions ; Relief high ; Interference color first order gray to yellow; Optically negative ( ) ; p. 341. 1 Chlorite, which is pleochroic, may appear uniaxial ; p. 352. 612 MINERALOGY 6. Double refraction greater than that of quartz. Scapolite; Tetragonal; n = 1.55-1.58; o> = .013- .035. Fibrous aggregates, rounded grains or rods; Cleavage prismatic; Colorless; Not pleochroic; Interference colors, bright, higher first and lower second orders; Optically negative ( ) ; p. 453. Calcite; Hexagonal; n = 3.57; o> - = .172. Irregular masses and grains, often showing twinning lamellae; Cleavage well developed, rhombohedral, 74 55'; Relief varies greatly with the direction of the section ; Colorless or pale ; Not pleochroic ; Inter- ference colors pale of high orders ; Optically ( ) ; p. 379. Tourmaline; Hexagonal; n = 1.65; o> = .017- .034. Lath-shaped, hexagonal or trigonal outlines, fibrous aggregates or rounded grains ; Cleavage none ; Color, greenish to dark brown, or pale; Pleochroism very marked, increasing with the depth of color of the section; Relief marked; Interference colors, upper first and lower second orders; Optically negative (-); p. 473. Zircon; Tetragonal; n = 1.96; - SO 4 ; Colorless to brownish; H. 2-3; G. 2.69; F. 1.5-2; IV; p. 527. d. It shows a potassium flame (violet), p. 563. a. With cobalt solution shows aluminium, p. 568. Kalinite, KA1(SO 4 ) 2 . 12 H 2 0;. Colorless; Vitreous; H. 2-2.5; G. 1.75; F. 1 ; I; p. 542. p. With copper oxide shows chlorine, a, p. 580. KAINITE. MgSO 4 .KC1.3 H 2 O ; Colorless ; H. 2.5-3 ; G. 2.13; F. 1.5-2; V ; p. 534. "y. With cobalt solution shows magnesia; a, p. 567. Picromerite, MgSO 4 .K 2 SO 4 .6 H 2 O ; White; H. ?; G. 2.15; F. 1.5-2; V; Yields water. Langbeinite, 2 MgS0 4 .K 2 SO 4 ; Colorless; H. 3-4; G. 2.81; F. 1.5-2; I; Yields no water. 8. With sodium hydroxide yields ammonia ; p. 565. Taylorite, K 5 (NH 4 )(SO 4 )3; Yellowish white; H. 2; Massive. . Shows calcium ; 6, p. 566. Syngenite, CaK 2 (SO 4 ) 2 .H 2 ; Colorless; H. 2.5; G. 2.60; F. 1.5-2; V. e. With cobalt solution shows aluminium ; p. 568. Alunogen, A1 2 (SO 4 ) 3 .18 H 2 O ; White ; H. 1.5-2; G. 1.70; V; Fibrous. Tschermigite, NH 4 A1(SO 4 ) 2 .12 H 2 O ; White; H. 1-2; G. 1.50; I ; Yields NH 3 with NaOH. /. With cobalt solution shows magnesium ; a, p. 567. + . With sodium hydroxide shows ammonia, p. 565. Boussingaultite, MgSO 4 . (NH 4 ) 2 S0 4 .6 H 2 ; Colorless ; G. 1.7; V. . It yields no ammonia. EPSOMITE, MgSO 4 .7 H 2 O ; White; Vitreous; H. 2-2.5; G. 1.75; F. 1 ; IV; p. 538. Kieserite, MgSO 4 .H 2 O ; White, gray, yellow; H. 3-3.5; G. 2.56; F. 2-3?; V; p. 539. g. Entirely volatile, boiled with NaOH shows ammonia. Mascagnite, (NH 4 ) 2 SO 4 ; White; Vitreous; H. 2-2.5 ; G. 1.77; F. 1; IV. h. Dissolved in borax on wire shows manganese ; a, p. 574. MINERALS HAVING TASTE 639 Mallardite, MnS0 4 .7H 2 O; Pink to white; V; Fibrous. Szmikite, MnSO 4 .H 2 O ; White to pink; H. 1.5; G. 3.15; Amorphous. Apjohnite, MnAl 2 (S0 4 ) 4 . 24H 2 O ; White to pale rose; Silky; H. 1.5; G. 1.78; V; Fibrous. i. Fused with soda in the R. F. on coal it yields a zinc coat. Goslarite, ZnS0 4 .7 H 2 O ; White; Vitreous; H. 2-2.5; G. 2.00 ; IV ; Acicular. B. Effervesces in dilute HCl (carbonates}. 1. Yields a strong sodium flame (yellow). TRONA, HNa 3 (CO 3 ) 2 .2H 2 O; White to gray; Vitre- ous; H. 2.5-3; G. 2.13; F. 1.5; V; p. 401. NATRON, Na 2 CO 3 .10H 2 0; White to gray; Vitreous; H. 1-1.5; G. 1.44; F. 1 ; V; p. 400. Thermonatrite, Na 2 C0 3 .H 2 O ; White, gray to yellow; H. 1-1.5; G. 1.55; F. 1.5; IV. C. With copper oxide shows chlorine; a, p. 588. 1. Yields a strong sodium flame. HALITE, Nad; White, red, blue ; H. 2.5; G. 2.13; F. 1.5; I; p. 327. 2. It yields a strong potassium flame. SYLVITE, KC1; White; H. 2; G. 1.98; F. 1.51; p. 328. With cobalt solution shows magnesium, p. 567. CARNALLITE, MgCl 2 ,KC1.6 H 2 ; White to red ; H. 1; G. 1.60; F. 1-1.5; IV; p. 335. 3. Yields a calcium flame. a. Shows magnesium with cobalt solution ; a, p. 567. Tachydrite, CaMg 4 Cl 6 .12 H 2 O ; Wax- to honey-yellow ; H. 2.5; F. 1; III. b. It contains no magnesium. Hydrophylite, CaCl 2 ; White; G. 2.2; F. 1.5; I. D. Heated in a closed tube with potassium bisulphate it yields orange-colored fumes (nitrates) . 1. It yields a yellow flame (sodium). SODA NITER, NaNO 3 ; White; Vitreous; H. 1.5-2; G. 2.29; F. 1.; Ill; p. 520. 2. It yields a violet flame (potassium). NITER, KNO 3 ; White; Vitreous; H. 2; G. 2.13; F. 1; IV; p. 521. 640 MINERALOGY 3. It yields a green flame (barium). Nitrobarite, Ba(NO 3 ) 2 ; White; Vitreous; H. 2.5; G. 3.20; F.l-1.5; I. E. With turmeric paper the HCl solution shows boric acid; b, p. 592. 1. In the closed tube yields water. BORAX, Na 2 B 4 3 .10 H 2 O ; Vitreous; H. 2-2.5; G. 1.72; F. 1-1.5; V; p. 522. Sassolite B(OH) 3 ; White; Pearly; H. 1 ; G. 1.48; F. 0.5; VI. II. Easily and completely volatile (when pure) in a gentle O. F. If the mineral decrepitates, it should be heated in a closed tube, when it should volatilize and yield a sublimate. A. The mineral burns with a blue flame, yielding sulphur dioxide. SULPHUR, S; Pale yellow ; Resinous ; H. 1.5-2.5; G. 2.07 ; F. 1 ; IV ; p. 285. B. With soda in R. F. on coal it yields an arsenic odor. 1. Fused with soda in R. F. it yields on silver a sulphur re- action. . Yields no green flame. REALGAR, AsS ; Aurora-red; Resinous; H. 1.5-2; G. 3.55; F. 1; V; p. 294. ORPIMENT, As 2 S 3 ; Lemon-yellow; Pearly to resin- ous; H. 1.5-2; G. 3.48; F. 1; V; p. 295. -f . It yields a green flame (thallium). Lorandite, T1 2 S. As 2 S 3 ; Carmine-red ; Adamantine ; H. 2-2.5 ; G. 5.53 ; F. 1 ; V. 2. It yields no sulphur reaction with soda. Arsenolite, As 2 O 3 ; White; Adamantine; H. 1.5; G. 3.70; Volatile; I; p. 346. Claudetite, As 2 3 ; White; Pearly; H. 2.5; G. 4.00; Volatile; V. C. Yields a white coat which colors the inner flame pale yellowish green (antimony); p. 584. 1. With soda it yields a sulphur reaction. KERMESITE, Sb 2 S 2 ; Brownish red to maroon; H. 1-1.5; G.4.60; F.I; V. 2. Yields no sulphur reaction with soda. Senarmontite, Sb 2 O 3 ; White ; Adamantine ; H. 2-2.5 ; G. 5.25; F. 1.5; I. COPPER MINERALS 641 VALENTINITE, Sb 2 O 3 ; White; Pearly, adamantine; H. 2.5-3; G. 5.56; F. 1.5; IV; p. 346. D. Fused with soda in the closed tube it yields mercury; a, p. 579. 1. Fused with soda it yields a sulphur reaction on silver. CINNABAR, MgS ; Red, vermilion ; Adamantine ; H. 2-2.5; G. 8.10; F. 1.5; III; p. 304. 2. With copper oxide it shows chlorine, a, p. 588. Calomel, HgCl ; White; Adamantine; H. 1-2; G. 6.48; Volatile; II. Eglestonite, Hg 4 Cl 2 O ; Yellow to black; H. 2-3; G. 8.327 ; I ; Darkens on exposure. Terlinguaite, Hg 2 ClO, Sulphur-yellow, dark olive; H. 2-3 ; G. 8.725 ; V. 3. With copper oxide shows no chlorine.' Montroydite, HgO ; Red; Adamantine; H. 1.5. E. Reduced with soda on coal, it yields lead buttons. Contunnite, PbCl 2 ; White; Adamantine; H. 1-2; G. 5.8; Volatile; IV. III. Powdered, roasted, and then reduced with soda and borax in R. F. on coal, it yields malleable buttons or scales when washed in the mortar. A. The button is copper or contains copper. 1. Dissolves in hot dilute HC1 with effervescence (carbonates). MALACHITE, (CuOH) 2 CO 3 , Bright green; Vitreous; H. 3.5-4; G. 3.96; F. 3 ; V; p. 397. AZURITE, Cu(Cu.O.H) 2 (CO 3 ) 2 ; Azure-blue ; Vitreous ; H. 3.5-4; G. 3.77; F. 3; V; p. 399. Aurichalcite, 2 (Zn.Cu)CO 3 . 3 (Zn.Cu)(OH) 2 ; Pale green to blue; Pearly; H. 2; G. 3.6; F. Dif. ; V. 2. The HC1 solution yields a white precipitate with barium chloride (sulphates). + . Completely soluble in water, when pure. CHALCANTHITE, CuSO 4 .5 H 2 O ; Azure-blue; Vit- reous; H. 2.5; G. 2.21; F. 3 ; VI; p. 540. Pisanite, (Fe.Cu)SO 4 .7H 2 O; Blue; Vitreous; H. 2.5; F. 3-4 ; V ; Becomes magnetic. Krohnkite, CuNa 2 (SO 4 ) 2 .2 H 2 O ; Azure-blue; H. 2.5; G. 1.98; F. 1; V; Yellow flame. Cyanochroite, CuK 2 (SO 4 ) 2 .6 H 2 O ; blue; Vitreous; V. Hydrocyanite, CuS0 4 ; Pale green, Brownish yellow ; F. 3; V; Yields no water. 2T 6 42 MINERALOGY . Not completely soluble in water. a. Alone yields an azure-blue flame. Spangolite, (A1.C1)S0 4 .6 Cu(OH) 2 .3 H 2 ; Dark green ; Cl. basal; H. 2-3 ; G. 3.14; F. 3 ; III. Connellite, Cui B (Cl.OH) 4 SOi6.15 H 2 ; Blue; H. 3 ; G. 3.36 ; F. 2.5 ; III. Prismatic. b. Do not yield an azure-blue flame. BROCHANTITE, Cu 4 (OH) 6 SO 4 ; Deep emerald-green; H. 3.5-4; G. 3.9; F. 3.5; IV; p. 541. Stelznerite, CuSO 4 .2Cu(OH) 2 ; Green; G. 3.88 ; IV. Langite, CuSO 4 .3 Cu(OH) 2 .H 2 O ; Blue, greenish blue; H. 2.5-3; G. 3.50; F. 3.5; IV. Herrengrundite, 2(Cu.OH) 2 S0 4 .Cu(OH) 2 .3 H 2 ; Em- erald-green; H. 2.5; G. 3.1; F. 3.5. ; V. Cyanotrichite, Cu 4 Al 2 SOi .8 H 2 ; Clear blue ; Pearly ; G. 2.7; F. 3; IV. Lindackerite, (Cu.OH) 4 Cu 2 Ni 3 (SO 4 ) ( As0 4 ) 4 .5 H 2 O ; Verdigris- to apple-green ; H. 2-2.5 ; G. 2.25 ; F. 2-3 ; IV. Dolerophanite, (Cu 2 0)SO 4 ; Brown; F. 3 ; V; Yields little or no water. c. With von KobelFs flux shows lead. Arzrunite, PbS0 4 .PbO, 3CuCl 2 Cu(OH) 2 .H 2 O ; Blue; F. 2; IV. Linarite, [(Pb.Cu)OH] 2 SO 4 ; Azure-blue; Cl. pina- coidal; H. 2.5; G. 5.45; F. 1.5; V. Caledonite, [(Pb.Cu)OH] 2 SO 4 ; Bluish green; Cl. basal; H. 2.5-3; G. 6.40; F. 1.5; IV. 3. The powdered mineral heated in the closed tube with a few small fragments of coal yields an arsenic mirror; 6, p. 585. a. The powdered mineral in R. F. becomes magnetic. Chenevixite, Cu 2 (Fe.O) 2 (AsO 4 ) 2 .3 H 2 O; Dark to olive- green ; Dull ; H. 4.5 ; G. 3.94 ; F. 2.5 ; Massive. b. With soda and borax in R. F. on coal yields a zinc oxide coat. Veszelyite,7(Cu.Zn)0.(As.P) 2 5 .9H 2 0; Greenish blue; H. 3.5-4; G. 3.79; VI. c. With von KobelPs flux shows bismuth ; 6, p. 580. Mixite, Cu 2 (Cu.OH) 8 Bi(AsO 4 ) 5 .7H 2 O; Pale green; H. 3-4; G. 3.8; F. 2; Capillary. COPPER MINERALS 643 d. With von Kobell's flux shows lead ; c, p. 579. Bayldonite, (Pb.Cu) 3 (AsO 4 ) 2 (Pb,Cu)(OH) 2 .H 2 O ; Grass- to black-green ; Resinous; H. 4.5 ; G. 5.35; F. 2-3 ; Mammillary. e. Fused with soda in R. F. on coal it yields a sulphur re- action; 6, p. 587. Lindackerite (Cu.OH) 4 Cu 2 Ni 3 S0 4 (As0 4 ) 4 .5 H 2 O; Ver- digris- to apple-green ; H. 2-2.5 ; G. 2-2.5 ; F. 2-3 ; IV. /. Yields a reaction for uranium ; 6, p. 576. Zeunerite, Cu(UO 2 ) 2 (AsO 4)2-8 H 2 O; Emerald-green; H. 2-2.5 ; G. 3.2 ; F. 2-3 ; II. g. Contains calcium, b, p. 566. Conichalcite, (Cu, Ca)(Cu.OH)(As.P)O 4 .i H 2 O; Em- erald-green; H. 4.5; G. 4.12; F. 2.5; Massive. Tyrolite, (Cu.Ca)(Cu.OH) 4 (AsO 4 ) 2 .7 H 2 O; Pale apple- green; H. 1-1.5; G. 3.05; F. 2.5; IV. h. Contains aluminium ; 6, p. 568. Liroconite, (Cu.OH) 9 Al 4 (OH) 6 , (AsO 4 ) 5 .20 H 2 0; Sky- blue; H. 2-2.5; G. 2.9; F. 3-3.5; V. i. Not included above, contains copper as the base. Clinoclasite, (Cu.OH) 3 AsO 4 ; Dark to bluish green; H. 2.5-3 ; G. 4.36 ; F. 2.5 ; V. OLIVENITE, Cu(Cu.OH), As0 4 ; Blackish olive-green to brown; H. 3 ; G. 4.4; F. 2.5; IV; p. 513. Euchroite, Cu(Cu.OH)AsO 4 .3 H 2 O ; Emerald-green; H. 3.5-4 ; G. 3.39 ; F. 2.5 ; IV. Chalcophyllite, (Cu.OH) 3 AsO 4 (Cu.OH) 2 .3i H 2 O ; Grass- green; Cl. basal; H. 2; G. 2.53; F. 2.5; III. Erinite, Cu(Cu.OH) 4 (As0 4 ) 2 ; Emerald-green; Dull; H. 4.5 ; G. 4.04 ; F. 2.5 ; Mamillary. Cornwallite, Cu(Cu.OH) 4 (AsO 4 ) 2 .3 H 2 ; Emerald- green; H. 4.5; G. 4.16; F. 2.5; Massive. Leucochalcite, Cu(Cu.OH)AsO 4 .H 2 ; White to pale green ; Silky ; F. 2.5 ; Capillary. Trichalcite, Cu 3 (As0 4 ) 2 .5 H 2 O ; Verdigris-green; Silky; H. 2.5; F. 2.5; Radiated and columnar. 4. The powdered mineral dissolved in nitric acid shows phos- phoric acid with ammonium molybdate ; b, p. 588. a. In R. F. it becomes magnetic. Chalcosiderite, Cu(Fe.Al) 2 (FeO) 4 (P0 4 ) 4 .8 H 2 O; Light 644 MINERALOGY to dark green; Cl. basal; H. 2.5 ; G. 3.1; F. 4-4.5; VI. b. With von KobelPs flux shows lead, p. 579. Vauquelinite, (Pb.Cu) 3 (P.O 4 ) 2 .2 (Pb.Cu)Cr0 4 ; Green, brown; Resinous; H. 2.5-3 ; G. 5.95; F. 2?; V. c. In S. Ph. shows uranium; a, p. 576. TORBERNITE, Cu(UO 2 ) 2 (P0 4 )2.8 H 2 O ; Emerald- to apple-green ; Cl. basal ; H. 2-2.5 ; G. 3.50 ; F. 3 ; II; p. 519. d. Phosphates of copper only. LIBETHENITE, Cu(Cu.OH)P0 4 ; Dark to olive-green; H. 4; G. 3.70; F. 2.5; IV; p. 513. PSEUDOMALACHITE, (Cu.OH) 3 PO 4 ; Dark to emer- ald-green; H. 4.5-5; G. 4-4.4; Massive. Dihydrite, Cu(Cu.OH) 4 (PO 4 ) 2 ; Dark emerald-green; H. 4.5-5; G. 4.20; F. 2.5; VorVI?. Tagilite, Cu(Cu.OH)P0 4 .H 2 O ; Emerald-green; H. 3-4; G. 4.07; F. 2.5; V; Fibrous. 5. Alone yields an azure-blue flame. +. With von Kobell's flux shows no lead. ATACAMITE, Cu 2 Cl(OH) 3 ; Deep emerald-green; H. 3-3.5 ; G. 3.75 ; F. 3-4 ; IV ; p. 334. Footeite, 8 Cu(OH) 2 .CuCl 2 .4 H 2 ; Deep blue ; V. Nantokite, CuCl ; White; Adamantine; H. 2-2.5; G. 3.93 ; F. 1.5 ; I. Yields no water. . With von Kobell's flux shows lead. Percylite, PbCuCl 2 (OH) 2 ; Indigo-blue; Brilliant; Cl. cubic ; H. 3 ; G. 5.08 ; F. 1 ; I. Cumengite, PbCuCl 2 (OH) 2 ; Indigo-blue; Cl. pyrami- dal ; H. 3 ; G. 4.71 ; F. 1 ; II. 6. Decomposed with HC1, leaving a residue of silica ; d, p. 592. CHRYSOCOLLA, CuSiO 3 .2 H 2 O ; Green to turquoise- blue ; H. 2-4 ; G. 2.12 ; F. Difficult ; Massive, p. 503. DIOPTASE, H 2 CuSiO 4 ; Emerald-green; Cl. rhombo- hedral ; H. 5 ; G. 3.35 ; F. Difficult ; III ; p. 452. 7. Not included above. CUPRITE, Cu 2 O ; Ruby-red ; Adamantine ; H. 3.5-4 ; G. 6.00; F. 3; I; p. 337. a. Fused with soda it yields a sulphur reaction. COVELLITE, CuS ; Indigo-blue; Cl. basal ; H. 1.5-2; G. 4.6; F. 2.5; III; p. 306. SILVER AND TIN MINERALS 645 b. Heated in a closed tube with potassium bisulphate it yields NO 2 ; a, p. 590. Gerhardtite, Cu 2 (OH)O 3 ; Deep emerald-green; Cl. basal; H. 2; G. 3.42; F. 3; IV. c. Heated as in b it yields iodine ; a, p. 589. Marshite, Cul ; Reddish brown ; Resinous; F. 1.5; I. Cuproiodargyrite, Agl.CuI ; Sulphur-yellow; H. 2; Massive. d. It yields a vanadium reaction ; 6, p. 577. Volborthite, (Cu.OH) 3 VO 4 .6 H 2 O ; Cl. pinacoidal ; H. 3-3.5; G. 3.55; F. 1.5; Tabular. Calciovolborthite, (Cu.Ca)(Cu.OH)VO 4 ; Pistachio- to olive-green; H. 3.5 ; G. 3.86 ; F. 1.5-2; Tabular. e. It yields a tungsten reaction ; b, p. 587. Cuprotungstite, CuW04 ; Pistachio-green ; Cl. pina- coidal ; H. 4.5-5 ; F. 3 ; Granular. /. It yields a reaction for selenium; a, p. 587. Chalcomenite, CuSe0 3 .2 H 2 O ; Blue; H. 2.5-3; G. 3.76; F. 1.5; V. B. Treated as in III, it yields silver buttons or it contains silver, but no Cu. 1. Alone in R. F. on coal it yields an arsenical odor. PROUSTITE, 3Ag 2 S.As 2 S 3 ; Ruby-red; Adamantine; H. 2-2.5; G. 5.55; F. 1 ; III; p. 323. Xanthoconite, 3 Ag 2 S.As<>S3 ; Orange-yellow to brown ; Cl. basal ; H. 2 ; G. 5.54 ; F. 1 ; V ; Tabular. 2. Yields in R. F. on coal an antimony coat. PYRARGYRITE, 3 Ag 2 S.Sb 2 S 3 ; Dark-red to black; H. 2.5; G. 5.85; F. 1; III; p. 322. Pyrostilpnite, 3Ag 2 S.Sb 2 S 3 ; Hyacinth-red ; H. 2 ; G. 4.20 ; F. 1 ; V ; Tabular. 3. With copper oxide it shows chlorine. CERARGYRITE, AgCl ; Gray to colorless; H. 2-3; G. 5.55 ; F. 1 ; I ; p. 330. lodobromite, Ag(Cl.Br.I) ; Sulphur-yellow to green; H. 2-3 ; G. 5.90 ; F. 1 ; I. 4. Shows bromine ; d, p. 589. EMBOLITE, Ag(Br.Cl); Green or yellow; H. 2-3; G. 5.80 ; F. 1 ; I ; p. 330. Bromyrite, AgBr ; Green to yellow ; H. 2-3 ; G. 5.90; F. 1 : I. 646 MINERALOGY 5. Shows iodine ; c, p. 589. IODYRITE, Agl ; Lemon-yellow; Resinous; H. 1.5; G. 5.65 ; F. 1 ; III ; p. 330. Miersite, Agl ; G. 5.64 ; F, 1 ; I. C. Treated as in III the button is tin. CASSITERITE, SnO 2 ; Brown to black; H. 6-7; G. 6.95; Inf.; II; p. 347. 1. Yields a boric acid flame with Turner's flux; p. 592. Nordenskiodine, CaSn(B.O 3 )2 ; Sulphur-yellow ; Cl. basal ; H. 5.5-6 ; G. 4.20 ; Inf. ; III. 2. Yields water in the closed tube. Stokesite, CaSn(Si0 3 ) 3 .H 2 O ; Colorless; H. 6; G. 3.18; IV. D. Treated as in III the button is lead, or with von Kobell's flux shows lead; 6, p. 580. 1. The powdered mineral heated with a few fragments of coal in the closed tube yields an arsenic mirror ; b, p. 585. a. Becomes magnetic in the R. F. Carminite, Pb 3 Feio(As0 4 )i2 ; Carmine-red; Cl, pris- matic; H. 2.5; G. 4.10; IV. Lossenite, (Fe.OH) 9 (As.O 4 )6.PbSO 4 .12 H 2 O ; Yellow to brownish red ; H. 3-4 ; F. 2.5 ; IV. Beudantite, Fe, Pb, Cu, SO 4 , (P.As)0 4 ?; Olive-green, brown to black; Cl. basal; H. 3.5-4.5; G. 4.15; F. 3.5; III. b. Yields a chlorine reaction; a, p. 588. MIMETITE, Pb 4 (Pb.Cl)(AsO 4 ) 3 ; White, yellow, brown ; Resinous; H. 3.5 ; G. 7.12; F. 1.5; III; p. 51?. Ecdemite, Pb 4 As 2 O 7 .2 PbCl 2 ; Yellow to green; Greasy; Cl. basal; H. 2.5-3 ; G. 7.00 ; F. 1.5?; IV. 2. Shows antimony, p. 584, and titanium; 6, p. 570. Mauzeliite, (Ca.Pb.Na2) 4 TiSb 4 Oi 6 ; Brown; H. 6-6.5 ; I. 3. Fused with soda it yields a sulphur reaction; 6, p. 587. a. Effervesces with hot nitric acid (carbonates). .Leadhillite, Pb 2 (Pb.OH) 2 (CO 3 ) 2 SO 4 ; White; Pearly; Cl. basal; H. 2.5; G. 6.45; F. 1.5; V. b. Yields a strong sodium flame (yellow). Caracolite, Pb(OH)ClNa2SO 4 ; White; H. 4.5; Fl. 5-2; IV. c. Becomes magnetic in R. F. LEAD MINERALS 647 Plumbojarosite, Pb[Fe(OH) 2 ]6(S0 4 )4 ; a dark brown powder; G. 3.66; III. d. Lead sulphates. ANGLESITE, PbS0 4 ; Gray to white ; H. 3 ; G. 6.35 ; IV; p. 532. Lenarkite, (Pb 2 0)SO 4 ; Pale yellow or white ; Cl. basal ; H. 2-2.5 ; G. 6.40 ; F. 2 ; V. e. Gelatinizes with HC1. Roeblingite, 5 H 2 CaSi0 4 .2 (Pb.Ca)SO 4 ; White; H. 2.5-3 ; G. 3.43 ; F. 3 ; Granular. 4. Effervesces, with hot dilute nitric acid (carborates). a. Yields a chlorine reaction; a, p. 588. Phosgenite, (PbCl) 2 CO 3 ; White; Adamantine; Cl. basal; H. 3; G. 6.2; F. 1; II. b. Yields water in the closed tubes. Hydrocerussite, Pb(Pb.OH) 2 (C0 3 ) 2 ; White; Pearly; H. 1-2; G. 6.14; F. 1-5; III. c. Yields no water. CERUSSITE, PbCO 3 ; White ; Adamantine ; H. 3-3.5 ; G. 6.55; F. 1.5;. IV; p. 396. Beresonite, 2 PbO.3 PbCr0 4 .PbC0 3 ; Dark red; Cl. perfect ; G. 6.69. 5. Yields an antimony coat in R. F. on coal. a. Yields a chlorine reaction; a, p. 588. Nadorite, PbClSb0 2 ; Smoky to yellowish brown; Resinous; H. 3.5-4; G. 7.0; F. 1.5; IV. Ochrolite, Pb 4 Sb 2 O7.PbCl2 ; Sulphur to grayish yellow ; F. 1.5?; IV. b. Yields no chlorine. Yields water. Bindheimite, Sb 2 O 3 .PbO and H 2 ; Gray, yellow, brown; H. 4 ; G. 4.68 ; F. 3-4 ; Amorphous. 6. The hot nitric acid solution yields a white precipitate with silver nitrate (chlorine); b, p. 588. Cotunnite, PbCl 2 ; White ; H. 2 ; G. 5.80 ; F. 1 ; IV. Penfieldite, 2PbCl 2 .PbO ; White ; Cl. basal ; H. 2.5 ; F. 1 ; III. Matlockite, PbCl 2 .PbO ; Pale yellow to white; Cl. basal ; H. 2.5-3 ; G. 7.20 ; F. 1 ; II. Mendipite, PbCl 2 .2 PbO ; Pale yellow to white ; Cl. prismatic; H. 2.5; G. 7.10; F. 1; IV. 648 MINERALOGY Laurionite, PbCl(OH); White; H. 3-3.5; F. 1; IV; Yields water. Nasonite, Pb 4 (Pb.Cl) 2 Ca4(Si 2 .0 7 )3; White; H. 4; F. 2 ; G. 5.42; II? In the closed tube yields a sublimate of lead iodide; d, p. 589. Schwartzenbergite, Pb(I.Cl) 2 .2 PbO ; Honey- to straw- yellow ; H. 2-2.5; G. 6.2; F. 1; III. 7. The nitric acid solution shows phosphoric acid with ammo- nium molybdate ; p. 590. PYROMORPHITE, Pb 4 (Pb.Cl)(P0 4 )3 ; White, yellow, brown, green; H. 3.5-4 ; G. 6.80 ; F. 2 ; III; p. 511. 8- The mineral powdered and dissolved in nitric acid shows silica; 6, p. 591. a. Becomes magnetic in R. F. Melanotekite, (Fe 4 O 3 )Pb 3 (Si0 4 ) 3 ; Dark brown to black; H. 5-5.5 ; G. 5.85 ; F. 2 ; IV. Hancockite, (Pb.Ca.Mn) 2 (Al.Fe.Mn.OH)(Al.Fe.Mn) 2 (Si.0 4 ) 3 ; Brownish red ; H. 6.5-7; G. 4.03; F. 3; V ; Shows Mn with borax. b. With Turner's flux it yields a green flame (boric acid). Hyalotekite, (Pb.Ca.Ba) 4 (F.OH)B(Si0 3 ) 6 ; White; H. 5.5; G. 3.80; F. 3?; Massive. c. In the borax bead it shows manganese. Kentrolite, (Mn 4 3 )Pb 3 (SiO 4 )3 ; Black; H. 5.5; G. 6.19 ; F. 2-2.5 ; IV. Ganomalite, Pb 3 Si 2 O 7 (Ca.Mn) 2 Si04 ; Gray to white; H. 3; G. 5.74; F. 3?; II. d. Silicate of lead only. Barysilite, Pb 3 Si 2 7 ; White ; Pearly ; Cl. basal ; H. 3 ; G. 6.50; F. 2.5; III. 9. Not included above. a. Shows molybdenum; a, p. 586. WULFENITE, PbMoO 4 ; Yellow, orange, red, white; H. 4.5-5; G. 6.05; F. 2 ; II; p. 545. , 6, Shows vanadium ; 6, p. 577. DESCLOIZITE, Pb(Pb.OH)VO 4 ; Brown, red; Resin- ous; H. 3.5; G. 6.05; F. 1.5; IV; p. 514. Brackenbuschite, Pb,Fe,Mn,(VO 4 ).H 2 O?; Dark-brown; F. 1.5; V. c. Shows vanadium, b, p. 577, and chlorine. ZINC AND BISMUTH MINERALS 649 Vanadinite, Pb 4 (Pb.Cl)(VO 4 ) 3 ; Ruby-red, brown, yel- low; H. 3; G. 6.83; F. 1.5; III. d. Shows tungsten ; 6, p. 587. STOLZITE, PbW0 4 ; Green, yellow, brown, red; Resinous ; H. 3 ; G. 8.00 ; F. 2.5 ; II ; p. 545. Raspite, PbWO 4 ; Wax-brown ; Resinous ; Cl. pina- coidal; H. 2.5-3 ; G.?; F. 2.5-3; V. e. Shows chromium ; b, p. 569. CROCOITE, PbCrO 4 ; Bright red; Adamantine; H. 2.5-3; G. 6.00; F. 1.5; V; p. 533. Phoenicochroite, 3 PbCrO 4 .PbO ; Red; Resinous; Cl. pinacoidal; H. 3-3.5 ; G. 5.75 ; F. 1.5; IV. /. Lead oxides only. Plattnerite, PbO 2 ; Brown-black; H. 5-5.5; G. 8.50; F. 1.5;. II. Minium,.' Pb 3 4 ; Red; Dull to greasy; H. 2-3; G. 4.60; F. 1.5; Pulverulent. Massicot, PbO ; Sulphur- to reddish yellow ; Dull ; H. 2; G. 8-9; F. 1.5; Massive. IV. Fused with soda and borax in the R. F. on coal, it yields a coat. A. It yields a zinc oxide coat; 6, p. 573. 1. Yields with soda a sulphur reaction; 6, p. 587. a. In R. F. becomes magnetic. Danalite, (Zn.Fe) 2 (Fe 2 .S)Be 3 (Si0 4 )3 ; Flesh-red to gray; H. 5.5-6 ; G. 3.43 ; F r 5. b. Dissolves in HC1, yielding H 2 S. SPHALERITE, ZnS ; White, green, yellow, brown, black; H. 3.5-4; G. 4.10; F. Difficult; I; p. 301. 2. Effervesces with HC1, yielding C0 2 (carbonates). SMITHSONITE, ZnC0 3 ; White, brown, green, pink; H. 5; G. 4.37; Inf.; Ill; p. 391. Hydrozincite, 2 ZnC0 3 .3 Zn(OH) 2 ; White, gray, yellow; H. 2-2.5 ; G. 3.69 ; Compact. 3. Gelatinizes with HC1. a. Yields little or no water. - WILLEMITE, Zn 2 SiO 4 ; White, yellow, green, blue; H. 5.5; G. 4.10; Inf.; Ill; p. 451. Hardystonite, Ca 2 ZnSi 2 7 ; White ; H. 3-4 ; F. Difficult ; G. 3.39; Yields a calcium flame. 6. Yields water. 650 MINERALOGY CALAMINE, (Zn.OH) 2 SiO 3 ; White, yellow, blue; H. 4.5-5; G. 3.45; IV; p. 472. Clinohedrite, H 2 CaZnSiO 5 ; Amethystine to white; H. 5-6 ; G. 3.33 ; F. 4 ; IV. Leucophoenicite, (Mn.Ca.Zn) 5 (Mn.Ca.ZnOH) 2 (SiO 4 )3 ; Purple-red; H. 5.5-6 ; G. 3.84 ; F. 3 ; V. 4. Not included above. a. Easily soluble in HC1. ZINCITE, ZnO; Deep red to orange-yellow; Cl. basal; H. 4-4.5 ; G. 5.55; Inf.; Ill; p. 339. b. The nitric acid solution shows phosphoric acid with ammonium molybdate. Hopeite, Zn 3 (PO 4 ) 2 .4 H 2 O ; Grayish white: Cl. pina- coidal; H. 2.5-3; G. 2.76; F. 3: IV. c. Not easily soluble in HC1. GAHNITE, ZnAl 2 O 4 ; Dark green ;H. 7.5-8; G. 4.55; Inf. ; I ; p. 377. JEFFERSONITE, (Ca.Mn)(Mg.Fe.Zn)(Si0 3 ) 2 ; Green- ish black to brown; Cl. prismatic: H. 5-6; G. 3.6; F. 4; V; p. 424. Disluite, (Zn.Fe.Mn.Mg)(ALFe)0 ; Green-black, green, gray; H. 7.5-8; G. 4.5; Inf.; I. B. With Von Kobell's flux shows bismuth ; p. 580. 1. The powdered mineral when heated in the closed tube with a few fragments of coal yields an arsenic mirror, p. 585. a. The S. Ph. bead in R. F. is green (uranium). Walpurgite, Bi ]0 .(UO 2 )3(OH) 2 4(As04)4; Wax-yellow; Cl. pinacoidal; H. 3.5 ; G. 5.76; F. 1.5; VI. b. The S. Ph is not green, or shows no uranium. Atelestite, (Bi.2 OH)(BiO) 2 AsO 4 ; Sulphur-yellow; H. 3-4; G. 6.40; F. 1.5; V. Rhagite, 3 Bi(OH) 3 .2 BiAs0 4 ; Yellow to greenish; Resinous; H. 5 ; G. 6.80; F. 1.5; Mammillary. 2. Effervesces with warm dilute HC1 (carbonates). a. Yields little or no water. Bismuthosphaerite, (Bi.O) 2 CO 3 ; White to gray ; Dull; H. 3-3.5; G. 7.42; F. 1.5; Massive. b. Yields water. Bismutite, (BiO)(Bi.2 OH)CO 3 ; Yellowish to grayish; Dull; H. 4-4.5; G. 6.88. ARSENIC MINERALS 651 3. Gelatinizes with HC1. Eulytite. Bi 4 (SiO 4 ) 3 ; Brown, yellow, colorless ; Resin- ous ; H. 4.5; G. 6.1; F. 2 ; I. Agricolite, Bi 4 (Si0 4 ) 3 ; Yellow, brown; H. 3?: G. 6.00; F. 2 ; V. 4. Soluble in HC1, shows vanadium ; p. 576. Pucherite, BiVO 4 ; Reddish yellow ; Cl. basal; H. 1; G. 6.25 ; F. 2 ; IV. 5. Shows tellurium. Montanite, (Bi.2 OH) 2 TeO 4 ; Yellow, green, white; H. 1.5? ; Dull ; Massive. C. Yields an antimony coat; a, p. 584. 1. The HC1 solution reduced with tin show r s titanium; 6, p. 570. Lewisite, (Ca.Fe) 5 Ti 2 Sb 6 24 ; Honey-yellow to brown; Resinous; H. 5.5; G. 4.95; I. 2. In R. F. becomes magnetic. Tripuhyite, Fe 2 Sb 2 7 ; Greenish yellow ; Resinous ; G. 5.82; F. 4-5?. 3. In R. F. not magnetic. Romeite, CaSb 2 4 ; Honey-yellow ; H. 5.5; G. 4.70; II. Manganostibnite, MnioSb 2 5 ; Black ; Compact. D. Yields a cadmium coat; a, p. 581. With soda a sulphur reaction. Greenockite, CdS ; Honey-, lemon- or orange- yellow ; H. 3-3.5 ; G. 4.95 ; III ; p. 306. V. In R. F. on coal yields an arsenical odor, or the powdered mineral heated with a few fragments of coal in a closed tube yields an arsenic mirror. A. In the borax bead it shows cobalt. ERYTHRITE, Co 3 (AsO 4 ) 2 .8 H 2 O ; Crimson to peach- red; Pearly; H. 1.5-2.5; G. 2.95; F. 2.5; V; p. 517. Forbesite, H(Ni.Co)As0 4 .3J H 2 ; Grayish white ; Silky; H. 2.5 ; G. 3.1 ; Fibrous. B. With borax shows nickel. Annabergite, Ni 3 (AsO 4 ) 2 .8 H 2 ; Apple-green ; H. 1.5-2.5; F. 4; V; Capillary. Cabrerite, (Ni.Mg) 3 (AsO 4 ) 2 .8H 2 O ; Apple-green ; Pearly ; H. 2 ; G. 2.96 ; F. 4-5 ; V. 652 MINERALOGY C. * The residue after treatment in R. F. is magnetic. The bead shows iron. 1. With soda in R. F. it yields a sulphur reaction. Pitticite, Fe, (As0 4 ), (SO 4 ).H 2 O?; Yellowish to red- dish brown ; H. 2-3 ; G. 2.35 ; Massive. 2. No sulphur, contains calcium ; b, p. 566. Arseniosiderite, [Fe 4 (OH) 6 ](Ca.OH)3(AsO 4 )3 ; Yellow- ish to golden brown; Silky; H. 1-2; G. 3.6; F. 3; Fibrous. Mazapilite, [Fe 4 (OH) 6 ]Ca8(AsO 4 )4.3 H 2 O ; Black to brownish red; H. 4.5; G. 3.57; IV. 3. Contains no sulphur or calcium, PHARMACOSIDERITE, Fe(Fe.OH) 3 (AsO 4 )3.6 H 2 O ; Green, yellow, brown, red; H. 2.5; G. 2.95; F. 1.5-2; I. SCORODITE, FeAs0 4 .2H 2 O; Pale green to brown; H. 3.5-4; G. 3.29; F. 2-2.5; IV. Symplesite, Fe 3 (AsO 4 ) 2 .8 H 2 ; Blue to mountain- green; Cl. Pinacoidal; H. 2.5; G. 2.95; F. Diffi- cult V. . D. Roasted and dissolved in borax on wire it shows manganese. 1. It dissolves in HC1. liberating chlorine. Synadelphite, 2(Mn.Al)AsO 4 .5 Mn(OH) 2 ; Brownish black to black ; H. 4.5 ; G. 3.47 ; F. 2-3 ; V. Flinkite, MnAsO 4 .Mn(OH) 2 ; Greenish brown; H. 4-4.5; G. 3.87; F. 2-3 ; IV. Hematolite, (Al.Mn)AsO 4 , 4Mn(OH) 2 ; brown to red; Cl. basal ; H. 3.5 ; G. 3.35 ; F. Difficult ; III. 2. Soluble in HC1 without liberating chlorine. Brandtite, Ca2Mn(AsO 4 ) 2 .2 H 2 ; White; H. 5-5.5; G. 3.67 ; F. 2.5-3 ; VI. Berzeliite, (Ca.Mg.Mn) 3 (As0 4 ) 2 ; Sulphur- to orange- yellow ; Resinous ; H. 5 ; G. 4.08 ; F. 3 ; I. Larkinite, Mn(Mn.OH) AsO 4 ; Flesh, rose, or yellowish red; Greasy; H. 4-5 ; G. 4.18; F. 2 ; V. Hemafibrite, Mn 3 (AsO 4 ) 2 .3 Mn(OH) 2 ; F. 2?; IV. Allactite, Mn 3 (AsO 4 ) 2 .4 Mn(OH) 2 ; Brownish red ; H. 4.5; G. 3.84; F. 2?; V. E. The roasted mineral in the S. Ph. bead shows uranium; p. 576. Trogerite, (U0 2 ) 3 (As0 4 ) 2 .12 H 2 O ; Lemon-yellow; Cl. pinacoidal ; G. 3.3 ; F. 2.5 ; V. PHOSPHATES 653 Uranospinite, Ca(U0 2 ) 3 (AsO 4 )2.8 H 2 ; Bright green ; Cl. basal; H. 2-3; G. 3.45; IV. F. Fused with potassium bisulphate in the closed tube, it shows fluorine. 1. It yields a sodium flame (yellow). Durangite, Na(AlF)AsO 4 ; Light to dark orange-red; C. prismatic ; H. 5 ; G. 4.0 ; F. 2 ; V. 2. Shows calcium ; 6, p. 566. Tilasite, Ca(MgF)AsO 4 ; Gray to violet; Cl. pina- coidal ; F. 4.5-5 ; Foliated. Svabite, Ca 4 (CaF)(As0 4 )3; Colorless; Cl. prismatic; G. 3.5; F. 4.5-5; III. G. The concentrated HCl solution yields a precipitate with a drop of sulphuric acid (calcium). Adelite, Ca(Mg.OH) As0 4 ; Gray; H. 5; G. 3.76; V. Haidingerite, HCaAs0 4 .H 2 O ; White; Pearly; Cl. pinacoidal; H. 1.5-2.5; G. 2.85; F. 2.5; IV. Pharmacolite, HCaAsO 4 .2 H 2 ; White or gray ; Pearly ; Cl. pinacoidal ; H. 2.5 ; G. 2.47 ; F. 2-3 ; V. H. Becomes violet with cobalt solution. Hoernesite, Mg 3 (As0 4 ) 2 .8 H 2 ; Snow-white ; Pearly ; Cl. pinacoidal ; H. 1 ; G. 2.47 ; F. 2-3 ; V. VI. The nitric acid solution shows phosphoric acid with ammonium molybdate, 6, p. 590. A. The powdered mineral treated with the R. F. on coal yields a magnetic residue. 1. Yields a lithium flame. TRIPHYLITE, Li(Fe.Mn)P0 4 ; Light blue, green, gray ; resinous ; Cl. basal ; H. 4.5 ; G. 3.55 ; F. 2.5 ; IV. 2. The borax bead shows manganese. a. Yields a strong yellow flame (sodium). Natrophilite, Na(Mn.Fe)P0 4 ; Deep wine-yellow; Resinous; Cl. basal ; H. 4.5-5 ; G. 3.41 ; F. 2-2.5 ; IV. Dickinsonite, (Mn.Fe.Ca.Na2) 3 (PO 4 ) 2 .i H 2 ; Olive-oil or grass-green ; Cl. basal ; H. 3.5-4 ; G. 3.34 ; F. 2.5; V. Fillowite, (Mn.Fe.Ca,Na 2 ) 3 (P0 4 ) 2 .J H 2 O ; Wax yellow to brown; Greasy; Cl. basal; H. 4.5; G. 3.43; F. 2.5-3; V. b. It will not yield a yellow flame. 654 MINERALOGY TRIPLITE, (Fe.Mn)(Mn.F)P0 4 ; Chestnut to blackish brown; Resinous; H. 4.6; F. 2.5; V. Yields water. Triploidite, (Mn.Fe)(Mn.OH)PO 4 ; Yellow to reddish brown; Cl. pinacoidal; H. 4.5-5; G. 3.70; F. 3 ; V. Childrenite, (Fe.Mn)(A1.2 OH)PO 4 .H 2 O ; Yellowish brown to brownish black ; Cl. pinacoidal ; H. 4.5-5 ; G. 3.20; F. 4; IV. Perpurite, 2(Mn.Fe)P0 4 .H 2 ; Reddish purple; H. 4-4.5; G. 3.15; F. 2. 3. The borax bead shows iron only. a. With soda it shows sulphur. Diadochite, 2(Fe.OH)SO 4 .2 FeP0 4 .H 2 O; Yellow or yel- lowish brown ; Resinous; H. 3 ; G. 2.03; F. 3?; V. 6. The concentrated HC1 solution shows calcium ; b, p. 566. Borickite, Ca(Fe.2 OH) 4 (P0 4 ) 2 .3 H 2 ; Reddish brown; Waxlike ; H. 3.5 ; G. 2.7 ; F. 3 ; Massive. Calcioferrite, Ca 3 (Fe.OH) 3 (P0 4 ) 4 .8 H 2 O ; Sulphur to greenish yellow ; Pearly; H. 2.5 ; G. 2.52; Massive. 4. Yields aluminum ; b, p. 568. Barrandite, (Al.Fe)P0 4 .2 H 2 O ; Pale blue, green, or yellow; H. 4.5 ; G. 2.57; Radiated. LAZULITE, (Mg.Fe)(AlOH) 2 (PO 4 ) 2 ; Azure-blue; Cl. prismatic ; H. 5-5.5 ; G. 3.06 ; F. Difficult ; V ; p. 438. 5. Phosphates of iron only. VIVIANITE, Fe 3 (PO 4 ) 2 .8H 2 O; Blue, bluish green, white; Cl. pinacoidal ; H. 1.52; G. 2.63; F. 2-2.5 ; V; p. 516. Ludlamite, Fe 5 (Fe.OH) 2 (P0 4 ) 4 .8H 2 0; Pale green ; Cl. basal; H. 3-4 ; G. 3.12; F. 2.5 ; V; Tabular. DUFRENITE, Fe 2 (OH) 3 PO 4 ; Dull olive- to black- green; Silky; H. 3.5-4; G. 3.31; F. 2.5; IV; p. 510. Beraunite, (Fe.OH) 3 (PO 4 ) 2 .2i H 2 O ; Reddish brown ; Cl. pinacoidal ; G. 2.95 ; F. 3 ; V; Foliated. Phosphosiderite, 2 FePO 4 .3J H 2 ; Pale red or red- dish violet ; Cl. pinacoidal ; H. 3-4 ; G. 2.76 ; F. 2.5; IV. Strengite, FePO 4 .2 H 2 ; Pale red or reddish violet ; H. 3-4 ; G. 2.87 ; F. 2.5 ; IV. PHOSPHATES 655 Koninckite, FePO 4 .3 H 2 ; Yellow; H. 3.5; G. 2.3; F. 2.5 ; Radiated. Cacoxenite, Fe 2 (OH) 3 P0 4 .4i H 2 O ; Golden-yellow ; Silky; H. 3-4 ; G. 3.38 ; F. 2.5 ; Radiated. B. The roasted residue is not magnetic. 1 . The borax bead shows manganese. Fairfieldite, Ca 2 Mn(PO 4 ) 2 .2 H 2 O ; White to greenish white; Cl. pinacoidal ; H. 3.5; G. 3.15; F. 4-4; 5; VI. Reddingite, Mn 3 (PO 4 ) 2 .3 H 2 O ; Pale rose to brown ; H. 3-3.5; G. 3.10; F. 2.5-3; IV. 2. The S. Ph. bead in R. F. is green (uranium). AUTUNITE, Ca(UO 2 ) 2 (PO 4 ) 2 .8 H 2 O ; Lemon, to sul- phur-yellow; Cl. basal; H. 2-2.5 ; G. 3.12; F. 3; IV ; Tabular, p. 520. Uranocircite, Ba(UO 2 ) 2 (P0 4 ) 2 .8 H 2 ; Yellowish green ; Cl. basal; H. 2-2.5 ; G. 3.53; F. 3?; IV; Tabular. Phosphuranylite, (UO 2 ) 3 (PO 4 ) 2 .6 H 2 O ; Deep lemon- yellow; Pearly; F. 3-4?; IV. C. The concentrated HCl solution yields a precipitate with sulphuric acid (calcium}. 1. Yields little or no water. APATITE, Ca 4 (CaF)(P0 4 ) 3 ; Green, blue, brown, white ; Cl. basal; H. 5 ; G. 3.15; G. 5-5.5; III; p. 508. 2. Yields water in the closed tube. HERDERITE, Ca[Be(OH.F)]PO 4 ; White, pale green or yellow ; H. 5 ; G. 3.00 ; F. 4 ; V. Hydro-herderite, Ca(Be.OH.F)PO 4 ; White, yellow, pale green ; H. 5 ; G. 2.95 ; F. 4 ; V. Cirrolite, (Ca.OH) 3 Al 2 (PO 4 ) 3 ; White, pale yellow; H. 5-6 ; G. 3.08 ; F. 4 ; Massive. Monetite, HCaPO 4 ; Yellowish white ; Cl. pinacoidal ; H. 3.5; G. 2.75; F. 3; VI. Collophanite, Ca 3 (P0 4 ) 2 .H 2 ; White, yellow; Dull; H. 2-2.5 ; G. 2.70 ; F. 4.5 ; Amorphous. Tavistokite, Ca 3 Al 2 (OH) 6 (P0 4 ) 2 ; White, pearly ; F. Diffi- cult ; Acicular. Svanbergite, (SO 4 )(P0 4 ), Al, Ca.H 2 O?; Yellow, brown, rose-red ; Cl. basal ; H. 5 ; G. 3.30 ; F. Difficult ; III. Hamlinite, (Sr.OH)(A1.2 OH) 3 P 2 O 7 ; White to yellowish white ; Cl. basal. H. 4.5 ; G. 3.23 ; F. 4 ; III. 656 MINERALOGY Martinite, H 2 Ca 5 (PO)4.H 2 O ; White, yellow; G. 3.9; 'F. Difficult ; III. 3. Yields much water. Isoclasite, Ca(Ca.OH)P0 4 .2 H 2 O ; White ; Cl. pina- coidal; H. 1.5; G. 2.92; F. 4? ; V. Brushite, HCaPO 4 .2 H 2 ; Colorless, pale yellow; Cl. ' pinacoidal ; H. 2-2.5 ; G. 2.20 ; F. 3 ; V. Goyazite, Ca 3 Al 10 P 2 O 2 3.9 H 2 ; Yellowish white; Cl. basal ; H. 5 ; G. 3.26 ; F. Difficult. D. Becomes blue with cobalt solution (aluminum) . Fusibility above 5. Variscite, A1P0 4 .2H 2 0; White, apple- to emerald- green ; Cl. prismatic ; H. 4 ; F. Difficult ; G. 2.4 ; IV. Callainite, AlP0 4 .2i H 2 ; Apple- to emerald-green ; H. 3.5-4; G. 2.51; Massive. Zepharovichite, A1P0 4 .3 H 2 ; Greenish to grayish green ; H. 5.5 ; G. 2.37 ; Compact. Minervite, AlPO 4 .3i H 2 O ; White ; Massive. GIBBSITE, A1P0 4 .4 H 2 O ; White; Massive; Foliated; p. 365. WAVELLITE, (A1.0H) 3 (P0 4 ) 2 .5 H 2 ; White, yellow, green, brown; H. 3-4 ; G. 2.33 ; IV; p. 510. Augelite, A1 2 (OH) 3 PO 4 ; White; Cl. prismatic; H. 4.5-5 ; G. 2.70 ; V. Peganite, Al 2 (OH) 3 P0 4 .li H 2 ; Dark to light green; H. 3-3.5; G. 2.50; IV. Fischerite, A1 2 (OH) 3 P0 4 .2J H 2 ; Grass- to olive-green ; H. 5; G. 2.46; IV. Sphaerite, A1 5 (OH) 9 (PO 4 ) 2 .12 H 2 ; Light gray or blue; H. 4 ; G. 2.53 ; Globular. Evansite, A1 3 (OH) 6 PO 4 .6 H 2 ; White, pale yellow or blue; H. 3.5-4; G.1.94; Massive. LAZULITE, (Mg.Fe)(A1.0H) 2 (P0 4 ) 2 ; Azure-blue; H. 5.5 ; G. 3.06 ; V ; p. 438. Florencite, Al 3 Ce(OH) 6 (P0 4 ) 2 ; Pale yellow; Greasy; Cl. basal ; G. 3.58 ; III. E. Not included above. 1. Heated in the closed tube yields the odor of ammonia. Struvite, NH 4 MgP0 4 .6 H 2 O ; White, yellow, brown; H. 2; G. 1.65; F. 3; IV. Stercorite, H(NH 4 )NaP0 4 .4 H 2 O ; White, yellow, brown; H. 2; G. 1.6; F. 1 ; V. IRON MINERALS 657 2. With Turner's flux it yields a boric acid flame (bright green). Liineburgite, Mg 3 (PO 4 ) 2 .B 2 3 .8 H 2 ; White; G. , 2.05 ; Fibrous, earthy. 3. Yields a strong sodium flame (yellow). Beryllonite, NaBePO 4 ; White; Cl. basal; H. 5.5-6; G. 2.84 ; F. 3-3.5 ; IV. 4. Yields a lithium flame (red). AMBLYGONITE, Li(Al.F)P0 4 ; W r hite, pale green, blue; Cl. basal; H. 6 ; G. 3.08 ; F. 2 ; VI; p. 512. Montebrasite, Li[Al(OH.F)]PO 4 ; White, blue, pale green; Cl. basal; H. 6 ; G. 3.00; VI. 5. The HC1 solution yields a precipitate of magnesium ammonium phosphate with ammonia. Wagnerite, Mg(MgF)P0 4 ; Pale yellow, gray, red; H. 5.5 ; G. 3.06 ; F. 3.5-4 ; V. Bobierrite, Mg 3 (P0 4 ) 2 .8 H 2 ; White; pinacoidal ; H. 2.5; G. 2.43; V. 6. Phosphates of the rare earths ; a, p. 575. MONAZITE, (Ce.La.Di)P0 4 ; Yellowish to reddish brown; H. 5-5.5 ; G. 5.10; V; p. 507. XENOTIME, YP0 4 ; Yellow to reddish brown; Cl. prismatic; H. 4-5 ; G. 4.53 ; Inf.; II. Rhabdophanite, (La.Di.Y.Er)P0 4 .H 2 O ; Brown, pink, yellow, white; H.3.5; G.3.95; F. Difficult; Massive. Churchite, Ca 3 Ceio(PO 4 )i 2 .24 H 2 0?; Smoky-gray, pink- ish; H. 3-3.5; G. 3.15. VII. The powdered mineral heated in R. F. on coal becomes magnetic. A. Effervesces in warm dilute HCl (carbonates). SIDERITE, FeC0 3 ; Light to dark brown; Cl. rhom- bohedral; H. 3.5-4 ; G. 3.85; Inf.; Ill; p. 388. Ankerite, Ca(Mg.Fe.Mn)(CO 3 ) 2 ; Gray, brown; Cl. rhombohedral ; H. 3.5-4 ; G. 3.03 ; Inf. ; III. Breunnerite (Mg.Fe)C0 3 , Brown, gray ; Cl. rhom- bohedral ; H. 3.5-4.5 ; G. 3 ; Inf. ; III. B. The 'finely powdered mineral is soluble in boiling concentrated HCl, leaving no residue of silica. 1. Yields little or no water. Hematite, Fe 2 3 ; ' Red to reddish black ; H. 5.5-6 ; G. 5.10; Inf.; Ill; p. 343. 2u (358 MINERALOGY 2. Yields water. LIMONITE, Fe 4 3 (OH)6; Yellow, brown to brownish black ; Silky, dull ; H. 5-5.5 ; G. 3.80 ; Inf. ; Mas- sive ; p. 363. Gothite, FeO(OH) ; Yellow, brown to brownish black ; Dull; H. 5-5; G. 4.37 ; Inf.; IV; p. 363. Xanthosiderite, Fe 2 0(OH) 4 ; Golden-yellow to brown; Silky, pitchy ; H. 2.5 ; Inf. ; Earthy, acicular. Turgite, Fe 4 O 5 (OH) 2 , Red to reddish brown ; Dull ; H. 5-6; G. '4.14; Inf.; Incrusted. a. Fused with soda it yields a sulphur reaction. Planoferrite, Fe 2 (OH) 4 S0 4 .13H 2 O ; Yellowish green ; H.3. Compare sulphates of iron, p. 635. b. After the separation of iron it shows magnesium ; c, p. 567. Pyroaurite, Fe(OH) 3 .3 Mg(OH) 2 .3 H 2 O ; Golden-yellow to silver-white ; Pearly; H. 2-3 ; Inf.; G. 2.07 ; III. C. Gelatinizes with HCl; p. 591. 1. Yields little or no water in the closed tube. ALLANITE, (Ca.Fe) 2 (Al.Ce.Fe) 2 (Al.OH)(Si0 4 )3 ; Brown to pitch-black ; Resinous ; H. 5.5-6 ; G. 3.90 ; F. 2.5 ; V ; p. 468. ILVAITE, CaFe 2 (Fe.OH)(SiO 4 ) 2 ; Iron-black; H. 5.5-6; G. 4.05; F. 2.5; IV; p. 472. FAYALITE, Fe 2 SiO 4 ; Yellow to dark yellowish green ; Resinous ; H. 6.5 ; G. 4.32 ; F. 4 ; IV ; p. 450. a. Fused with soda and niter on wire, shows manganese ; 6, P- 574. Hortonolite, (Fe.Mg.Mn) 2 SiO 4 ; Yellow to dark yel- lowish green ; Resinous ; H. 6.5 ; G. 4.03 ; F. 4.5 ; IV. Kenebelite, (Fe.Mn.Mg) 2 SiO 4 ; Gray, brown, green; Greasy; Cl. pinacoidal; H. 6.5; G. 3.95; IV. 2. Yields water in the closed tube. Cronstedtite, H 8 Fe"4Fe'" 4 Si 3 2 o ; Black to brownish black ; Cl. basal ; H. 3.5 ; G. 3.35 ; F. 4 ; III. Thuringite; HisFesCAl.Fe^SieOu ; Olive to pistachio- green; Dull; H. 2.5; G. 3.18; F. 4 ; Compact. D. Decomposed in HCl without gelatinizing, leaving a residue of silica; d, p. 591. 1. Fused with soda and niter on wire it shows manganese, b, p. 574. IRON MINERALS 659 Pyrosmalite, H 7 (Fe.Cl)(Fe.Mn) 4 (SiO 4 )4 ; Olive-green to brown; Cl. basal ; H. 4-4.5 ; G. 3.16; F. 3 ; III. ASTROPHYLLITE, (K.Na.H) 4 .(Fe.Mn.Mg.Ca) 4 Ti- (SiO 4 ) 4 ; Bronze to golden-yellow; Cl. pinacoidal H. 3 ; G. 3.35 ; F. 3 ; IV. 2. Shows no manganese. LEPIDOMELANE, (K.H) 2 Fe" 2 (Fe.Al) 2 (SiO 4 ) 3 ; Black to greenish black; Cl. basal; H. 3 ; G. 3.1; F 4.5-5; V. ANDRADITE, Ca 3 Fe 2 (SiO 4 ) 3 ; Wine, yellowish, green, brown; H. 7 ; G. 3.85 ; F. 3.5 ; I; p. 444. Stilpnomelane, H 6 (Fe.Mg) 2 (Fe.Al)2Si5O 18 ; Greenish to yellowish bronze; H. 3 ; G, 2.75; F. 4.5 ; Foliated. Hisingerite, H, Fe, Fe, Mg, Si, O?; Black to brown- ish black; H. 3; 2.75; F. Difficult ; Amorphous. Chloropal, H 6 Fe 2 (SiO 4 ) 3 .2 H 2 O ; White, olive, blackish, yellowish green; H. 2.5-3; G. 1.87; F. Difficult; Amorphous. E. Insoluble or only slightly attacked by HCl. 1. Yields little or no water. a. In S. Ph. an emerald green bead (chromium). CHROMITE, FeCr 2 4 ; Brownish black ; H. 7.5-8: G. 4.45 ; Inf. ; I ; p. 376. 6. The S. Ph. bead reduced with tin on coal, then dissolved in dilute HCl, shows tungsten, 6, p. 587. WOLFRAMITE, (Mn.Fe)WO 4 ; Brown to black; Cl. pinacoidal ; H. 5-5.5 ; G. 7.35 ; F. 4 ; V ; p. 542. c. Fuses with intumescence and colors the flame yellow (sodium). ARFVEDSONITE, (Fe.Na 2 .Ca) 4 (Si0 3 ) 4 .Fe 2 (Al.Fe)>- Si 2 Oi 2 ; Black; Cl. prismatic; H. 6 ; G. 3.45; F. 3.5; V; p. 419. Crocidolite, NaFe(Si0 3 ) 2 (Fe.Mg.Ca)SiO 3 ; Lavender blue; H. 6-6.5; G. 3.43 ; F. 3.5 ; Fibrous. Riebeckite, 2 NaFe(SiO 3 ) 2 .(Fe.Ca)SiO 3 ;' Black; Cl. prismatic; H. 6-6.5 ; G.3.43; F. 3 ; V. JEnigmatite, (Fe.Mn)(Fe.Al).Na, (Ti.Si)O?; Black, Cl. prismatic ; H. 6 ; G. 3.75 ; F. 3 ; VI. d. Fuses quietly, or with difficulty. ALMANDITE, Fe 3 Al 2 (SiO 4 ) 3 ; Deep to brownish red ; H. 7-7.5; G. 4.15; F. 3 ; I; p. 444. 660 MINERALOGY Babingtonite, (Ca.Mn.Fe)Si03.Fe 2 (SiO 3 )3; Greenish black to black; H. 5.5-6 ; G. 3.36 ; F. 3-3.5; VI. ACMITE, NaFe(SiOs)2; Greenish to brownish black; Cl. prismatic ; H. 6-6.5 ; G. 3.50; V. HYPERSTHENE, (Mg.Fe)Si0 3 ; Greenish black to bronze-brown; Cl. pinacoidal; H. 5-6; G. 3.45; F. 5; IV; p. 421. 2. Yields water. BIOTITE, (K.H) 2 (Mg.Fe)2(Al.Fe)2(Si0 4 )s ; Green to greenish black; C. basal; H. 2.5-3; G. 2.90; F. 5; V; p. 492. Chloropal, H 6 Fe 2 (SiO 4 )3.2 H 2 ; Greenish yellow to pistachio-green; H. 2-4; G. 3.87; F. Difficult; Amorphous. Compare minerals containing small amounts of iron in sections below, some of which may at times become magnetic. VIII. After intense ignition in the forceps, it yields an alkaline reaction with turmeric paper, hardness below 5. .4. Effervesces with hot dilute HCl, and dissolves, leaving little or no residue when pure (carbonates). 1. After intense ignition in the forceps it yields a green flame (barium). WITHERITE, BaC0 3 ; White to gray; H. 3.5 ; G. 4.3 ; F. 2.5-3; IV; p. 394. BARYTOCALCITE, BaCa(C0 3 ) 2 ; White, gray, yellow, green ; Cl. prismatic ; H. 4 ; G. 3.65 ; F. Difficult ; V ; p. 395. Bromlite, (Ca.Ba)C0 3 ; White, gray, cream ; H. 4^.5; G. 3.72 ; F. Difficult ; IV. 2. It yields a red flame (strontium). STRONTIANITE, SrCO 3 ; White, gray, yellow, green; Cl. prismatic ; H. 3.5-4 ; G. 3.70; IV; p. 395. 3. After intense ignition it yields a carmine flame (calcium). CALCITE, CaCO 3 ; White and variously tinted; Cl. rhombohedral ; H. 3 ; G. 2.72 ; Inf. ; III ; p. 379. ARAGONITE, CaCO :5 ; White and variously tinted; Cl. pinacoidal ; H. 3.5-4 ; Inf. ; IV ; p. 392. DOLOMITE. CaMg(C0 3 ) 2 ; White and variously tinted ; Cl. rhombohedral ; H. 3-4 ; G. 2.85 ; Inf. ; III ; p. 386. a. Yields a fluorine reaction ; a, p. 589. MINERALS ALKALINE AFTER IGNITION 661 Parisite, Ce(CeF)(CaF)(CO 3 ) 3 ; Yellowish brown to brown; Cl. basal ; H. 4.5 ; Inf; G. 4.36; III. b. In the closed tube yields water. Thaumasite, CaCO 3 .CaSiO 3 CaSO 4 .15 H 2 ; White; H. 3.5; G. 1.87; F. Difficult; III; Fibrous. c. In S. Ph. a green bead (uranium). Uranothallite, Ca 2 U(C0 3 ) 4 .10 H 2 O; Yellowish green; H. 2.5-3 ; IV ; Tabular. 4. Upon ignition in the forceps it yields an intense yellow flame (sodium). a. In the closed tube it yields water, GAYLUSSITE, Na 2 Ca(CO 3 ) 2 .5 H 2 O ; White to gray; Cl. prismatic; H. 2-3 ; G. 1.99; F. 1.5; V; p. 401. Dawsonite,, Na(A1.2 OH)CO 3 ; White; H. 3.5; G. 2.40 ; F. 4.5-5 ; V ; Radiated. Pirssonite, Na 2 Ca(C0 3 ) 2 .2 H 2 O ; White to gray ; H. 3-3.5; G. 2.35; F. 1.5; IV. 6. In the closed tube it yields no water. The nitric acid solution yields a precipitate with AgNO 3 (chlorine). Northupite, MgNa2(CO 3 ) 2 Cl ; Colorless to brown; H. 3.5-4 ; G. 2.38 ; F. 1 ; I. Fused with soda it yields a sulphur reaction. Tychite, MgNa 6 (C0 3 ) 4 S0 4 ; Colorless; H. 3.5; G. 2.58 ; F. 1 ; I. 5. It contains magnesium as the base ; a, p. 567. a. It yields no water in the closed tube. MAGNESITE, MgC0 3 ; White, gray, yellow, brown; Cl. rhombohedral ; H. 3.5-4.5; G. 3.06; Inf.; Ill; p. 385. Breunnerite, (Mg.Fe)C0 3 , Brown, gray; Cl. rhombo- hedral ; 3.5-4.5 ; G. 3 ; Inf. ; III ; p. 385. 6. Yields water. Hydromagnesite, Mg 2 (Mg.OH) 2 (CO 3 ) 3 .3 H 2 ; White; H. 3.5; G. 2.15; Inf.; V. Lansfordite, Mg 2 (Mg.OH) 2 (CO 3 ) 3 .21 H 2 O ; White; H. 2.5; G. 1.54; Cl. basal; Inf.; VI. Hydrogioberite, (Mg.OH) 2 CO 3 .2 H 2 ; White ; Cl . prismatic; G. 2.16; Inf.; Compact. Nesquehonite, MgCO 3 .3 H 2 O ; White ; Cl. prismatic ; H. 2.5; G. 1.84; Inf.; IV. 662 MINERALOGY B. Fused with soda and coal dust in the R. F. on coal it yields a sulphur reaction on silver; 6, p. 587. 1. Yields a barium flame (green). BARITE, BaSO 4 ; White, blue, yellow, red; Cl. basal and prismatic ; H. 3.5 ; G. 4.5; F. 4 ; IV; p. 528. 2. Yields a red flame (strontium). CELESTITE, SrSO 4 ; White, blue, red; Cl. basal, prismatic ; H. 3-3.5 ; G. 3.97 ; F. 3.5-4 ; IV ; p. 530. 3. It yields a calcium flame (carmine), or a calcium reaction; a, p. 566. a. Yields no water. ANHYDRITE, CaSO 4 ; White, blue, red, gray ; Cl. 3 pinacoidals ; H. 3-3.5 ; G. 2.95 ; F. 3-3.5 ; IV ; p. 531. Glauberite, CaNa^SO^ ; White to gray ; Cl. Basal ; H. 2.5-3; G. 2.75; F. 1.5-2; V. Oldhamite, CaS ; Pale chestnut-brown; Cl. cubic; H. 4;. G. 2.58; F. Difficult ; I. b. Yields water. GYPSUM, CaS0 4 .2H 2 O; White, gray, reddish; Cl. pinacoidal ; H. 2 ; G. 2.32 ; F. 3 ; V ; p. 536. c. Yields a yellow flame (sodium). Whattevillite, CaNa2(SO 4 ) 2 .4 H 2 O ; White ; Silky ; G. 1.81; F. 1.5-2; Acicular. d. Yields a potassium flame through the blue glass. Polyhalite, Ca,MgK 2 (SO 4 ) 4 .2 H 2 O ; Brick-red to yel- low; Cl. pinacoidal; H. 2.5-3; G. 2.77; F. 2; V?. Syngenite, CaK 2 (SO 4 ) 2 .2 H 2 O ; White ; Cl. pinacoidal ; H. 2.5; G. 2.60; F. 1.5-2; V. Ettringite, (Ca.OH) 6 (SO 4 ) 3 .2 A1(OH) 3 .24 H 2 O ; White ; H. 2-2.5; G. 1.75; F. 3 ; III. C. Fused with potassium bisulphate in the closed tube yields a flourine reaction; a, p. 589. 1. Yields little or no water. a. Will not become blue with cobalt solution. FLUORITE, CaF 2 ; White, green, yellow, violet, pink ; Cl. Octahedral ; H. 4 ; G. 3.18 ; F. 3 ; I ; p. 331. 6. Becomes blue with cobalt solution. CRYOLITE, Na 3 AlF 6 ; White to brownish ; Cl. pina- coidal ; H. 2.5; G. 2.97; F. 1.5; V; p. 333. Chiolite, 5NaF.3AlF 3 ; Snow-white; H. 3.5-4; G. 2.95; F. 1.5; II. BORATES 663 2. Yields water. Thomsenolite, NaCaAlF 6 .H 2 ; White to brown; Cl. basal; H. 2; G. 2.93; F. 1.5; V. Pachnolite, NaCaAlF 6 .H 2 ; White ; H. 3 ; G. 2.98 ; F. 1.5; V. Gearksutite, CaF 2 Al(F.OH) 3 .H 2 O ; White; Dull; H. 2; F. 1.5; Earthy. Prosopite, CaF 2 .2Al(F.OH) 3 ; White to gray; Cl. prismatic ; H. 4.5 ; G. 2.80 ; F. Difficult ; V. D. Not included above. 1. Becomes pink with cobalt solution (magnesia). a. Yields water. BRUCITE, Mg(OH) 2 ; White, gray, green ; Pearly ; Cl. basal; H. 2.5 ; G. 2.39 ; Inf.; Ill; p. 362. TALC, H 2 Mg 3 (Si0 3 ) 4 ; Apple-green, gray, white; Pearly; Cl. basal; H.I; G. 2.80; F. 5 ; Foliated. 6. Yields no water. Periclase, MgO ; White, gray, dark green ; Cl. cubic ; H. 5.5 ; G. 3.8 ; Inf. ; I. 2. Becomes blue with cobalt solution. Hydrotalcite, Mg 3 Al(OH) 6 .3H 2 O ; White; Pearly; Cl. basal ; H. 2 ; G. 2.07 ; Inf. ; III. 3. With Turner's flux it yields a boric acid flame (bright green). ULEXITE, NaCaB 5 9 .8H 2 O ; White; Silky; H. 1, G. 1.55; F. 1.5; Fibrous; p. 524. 4. Heated in the closed tube yields iodine. Dietzeite, 7 Ca(I0 3 ) 2 .8 CaCrO 4 ; Golden-yellow ; H. 3-4 ; G. 3.70 ; F. 1.5 ; V. Green in S. Ph. Lautarite, Ca(I0 3 ) 2 ; Sulphur-yellow to colorless ; Cl. prismatic; N. 3.5-4 ; G. 4.59 ; F. 1.5; V. Some silicates may contain enough calcite to yield an alkaline reaction after ignition; in general their hardness will be above 5 and are not placed in this group ; compare the silicates beyond. IX. With Turner's flux it yields a bright green flame (boric acid). A. Soluble in HCl 1. Yields little or no water. BORACITE, Mg 7 Cl 2 Bi 6 03o ; White, gray, green, brown; H. 7 ; G. 2.95 ; F. 3 ; I ; p. 522. Rhodizite, K(A1O) 2 (BO 2 ) 3 ; white ; H. 8 ; G. 3.41 ; F. 4.5; I. 664 MINERALOGY Pinakiolite, 3 MgB 2 O 4 .Mn"Mn'" 2 O 4 ; Black ; Cl. pin- acoidal ; H. 6 ; G. 3.88 ; F. 5 ; IV. 2. Yields water. a. In the borax bead shows manganese. Sussexite, H(Mn.Mg.Zn)BO 3 ; Gray; Silky; H. 3; G. 3.12; F. 2.5; IV?; Fibrous. 6. The concentrated HC1 solution yields a precipitate with H 2 SO 4 (calcium) ; 6, p. 566. COLEMANITE; Ca 2 B 6 O n .5 H 2 O ; White ; Cl. pina- coidal; H. 4-4.5 ; G. 2.42 ; F. 1.5; V: p. 524. Hydroboracite, CaMgB 6 O n .6 H 2 O ; White ; H. 2 ; G. 1.95; F. 1.5; Fibrous, foliated. Mr i- Bechilite, CaB 4 O 7 .4 H 2 O ; Massive. c. Yields magnesium reaction a, p. 567. Szaibelyite, Mg 5 B 4 On.li H 2 O ; White to yellow; H. 3-4; G. 3.0; Nodular, acicular. Pinnoite, MgB 2 O 4 .3 H 2 O ; Sulphur to straw-yellow; H. 3-4 ; G. 3.3 ; F. 3 ; II. Heintzite, KMg 2 B n O 19 .7 H 2 ; White; Cl. pinacoidal ; H. 4-5; G. 2.13; F. 1 ; V. B. The finely powdered mineral gelatinizes with HCL DATOLITE, Ca(B.OH)Si0 4 ; White, pale green, yellow ; H. 5-5.5; G. 2.95; F. 2.5; V; p. 463. Bakerite, 8 CaO, 2 B 2 O 3 , 6 SiO 2 .6 H 2 O ; White to pale green; Dull; H. 4.5 ; G. 2.13; F.I; Massive. C. Insoluble in HCl or only slightly soluble. 1. Yields water. Howlite, H 5 Ca2B 5 Si0 14 ; White; H. 3.5; G. 2.59; F. Difficult ; Nodular, fibrous. Hambergite, Be(Be.OH)BO 3 ; Grayish white ; Cl. pin- acoidal ; H. 7.5 ; G. 2.35 ; Inf ; IV. 2. Yields little or no water. AXINITE, H 2 (Ca.Fe.Mn) 4 (BO)Al 3 (SiO 4 ) 5 ; Clove- brown, green, yellow, gray ; Cl. pinacoidal ; H. 6.5-7 ; G. 3.29; F. 2.5-3; VI; p. 469. TOURMALINE, complex borosilicate ; Various colors ; H. 7-7.5 ; G. 3.14 ; F. 3-5 ; III ; p. 473. Danburite, CaB 2 (SiO 4 ) 2 ; White to pale yellow ; H. 7 ; G. 3.00; F. 3.5-4; Nodular, fibrous. Homilite, (Ca.Fe) 3 (BO) 2 (SiO 4 ) 2 ; Brownish black to black ; Resinous ; H. 5 ; G. 3.38 ; F. 2 ; V. MANGANESE MINERALS 665 Cappelenite, BaY 6 B 6 Si30 2 5; Greenish brown ; H. 6-6.5; G. 4.41; F. 4-5; III. a. Yields a titanium reaction b, p. 570. Warwickite, (Mg.Fe) 4 TiB 2 9 ; Hair-brown ; Dull ; Cl. pinacoidal ; H. 3-4 ; G. 3.36 ; Inf. ; IV. b. Becomes blue with cobalt solution. Jeremejevite, A1BO 3 ; White to pale yellow; H. 6.5; G. 3.28; Inf.; III. 3. Contains the rare earths; a, p. 572. Melanocerite, Si, Ta, B, Ce, La, Di, Y, Ca, Na, H, F, ? ; Deep brown to black ; Inf. ; Greasy ; H. 5-6 ; G. 4.13; III. Caryocerite, Si, Ta, B, Th, Ce, La, Di, Y, Ca, Na, H, F; Nut-brown; Greasy; H. 5-6; G. 4.29; Inf.; III. X. The finely powdered mineral dissolved in borax on wire in con- siderable quantities in the O.F. is violet-red when cold (manganese). A . The borax bead is powdered, dissolved in 1 cc. of dilute HCl and the solution is reduced with tin when it becomes blue; b, p. 587. HUBNERITE, MnWO 4 ; Brown to brownish black ; Resinous ; Cl. pinacoidal ; H. 5-5.5 ; G. 7.2 ; F. 4 ; V ; p. 542. Mangano-columbite, MnCb 2 6 ; Reddish to dark brown ; Resinous; Cl. pinacoidal ; H. 5 ; G. 6.6; Inf; IV. B. The solution is green (vanadium). Ardennite, I^M^AUVSi^s? ; Yellow to yellowish brown; Resinous; Cl. pinacoidal ; H. 6-7 ; G. 3.65; F. 2-2; IV. C. The solution is colorless or nearly so. 1. Fusibility below 5. or. Gelatinizes with HCl. . Yields no water. ! TEPHROITE, Mn 2 SiO 4 ; Smoky-gray to brownish red ; Cl. pinacoidal ; H. 5.5-6 ; G. 4.04; F. 3.5 ; IV. Glaucochroite, CaMnSi0 4 ; Blue, green; H. 6 ; G, 3.40; F. 3.5; IV. The HNO 3 solution yield a precipitate with silver nitrate (chlorine). Eudialyte, Si, Al, Ca, Na, K, O, Cl, (S0 4 )(CO 3 )?; 666 MINERALOGY ! Rose to brown; Cl. basal; H. 5-5.5; G. 2.92; F. 3 ; III. -K Yields water. Ganophyallite, Mn 7 (AlO) 2 (SiO 3 )8.6 H 2 ; Brown; Cl. basal; H. 4-4.5 ; G. 2.84; F. 3 ; V; Foliated. Dissolves in HC1, liberating hydrogen sulphide. Helvite, (Mn.Fe) 2 (Mn 2 S)Be 3 (S: O 4 ) 3 ; Yellow, brown, green, red; H. 6-6.5 ; G. 3.20; F. 4-4.5; I. b. Decomposed in HC1 without gelatinizing, -f . Yields water. Friedelite, H 7 (MnCl)Mn 4 (Si0 4 )4; Rose-red; Cl. basal; H. 4.5; G. 3.07; F. 4; III. Bementite, H 2 MnSi0 4 ; Pale grayish yellow ; Cl. basal ; H. 2.5-3 ; G. 2.98 ; F. 3.5 ; Foliated. Inesite, (Mn.Ca) 2 (Si0 3 ) 2 .H 2 O ; Rose- to flesh-red; Cl. pinacoidal ; H. 6 ; G. 3.03 ; F. 3 ; VI. . Yields no water. Trimerite, Be(Mn.Ca.Fe)SiO 4 ; White to salmon-pink; Cl. basal; H. 6-7 ; G. 3.47 ; F. 4-5?; VI. Dissolves in HC1 yielding hydrogen sulphide. ALABANDITE, MnS ; Iron- to greenish-black; H. 3.5-4; G. 3.95; F. 3 ; I; p. 304. c. Insoluble in HC1. -f. Yields water. Piedmontite, Ca,(Al.OH) 2 (Al.Mn.Fe) 2 (Si0 4 ) 3 ; Reddish brown; Cl. basal; H. 6.5; G. 3.5; F. 3; V; p. 467. Carpholite, Mn(A1.2 OH) 2 (SiO 3 ) 2 ; Straw- to wax-yellow ; Silky ; H. 5-5.5 ; G. 2.93 ; F. 3 ; V ; Fibrous. ' . Yields no water. Neptunite, (Na.K) (Fe.Mn) TiSi 4 O 12 ; Black; Cl. pris- matic ; H. 5-6 ; G. 3.23 ; F. 3.5 ; V. SPESSARTITE, Mn 3 Al 2 (Si0 4 ) 3 ; Brownish to garnet- red; H. 7-7.5; G. 4.2; F. 3 ; I; p. 444. Partschinite, (Mn.Fe) 3 Al 2 (SiO 4 ) 3 ; Yellowish red ; Greasy ; H. 6.5-7; G.4.00; F. 3 ; V. RHODONITE, MnSiO 3 ; Rose-red, pink, brown; Cl. prismatic; H. 6-6.5-; G. 3.63 ; F. 3.5 ; VI; p. 430. Schefferite, (Ca.Mn)(Mg.Fe)(SiO 3 ) 2 ; Yellowish to red- dish brown; Cl. prismatic; H. 5-6; G. 3.5; F. 4; V. TITANIUM AND TUNGSTEN MINERALS 667 Richterite, (Mg.Mn.Ca.Na 2 )4(Si0 3 ) 4 ; Brown, yellow, rose-red; Cl. prismatic ; H. 5.5-6; G.3.09; F. 4; V. 2. Fusibility above 5. a. Effervesces with warm dilute HC1 (carbonates). RHODOCROSITE, MnC0 3 ; Rose to dark red, brown; Cl. rhombohedral ; H. 2.5-4.5 ; G. 3.52 ; III ; p. 390. Torrensite, MnSiO 3 .MnC0 3 .i H 2 ; Reddish gray ; Compact. b. Do not effervesce in HC1. + . Yields water. WAD, Impure oxides of manganese; Gray, brown, black; Dull; Massive; Earthy; p. 368. Pyrochroite, Mn(OH) 2 ; White to bronze; Pearly; Cl. basal ; H. 2.5 ; G. 3.26 ; III. . Yields no water. Manganosite, MnO ; Dark emerald-green; Cl. cubic; H. 5-6; G. 5.18; I. XI. The mineral, well powdered, is dissolved in considerable quantities in the borax bead on wire. The bead is then powdered and dissolved in 1 cc. strong HC1 ; to the hot solution powdered tin is added. I . The solution is violet (titanium) . // the solution is poured off the tin and a few drops of hydrogen peroxide added, it be- comes a deep orange or yellow according to the quantity of titanium present. 1. Gelatinizes with HC1. Fusibility below 5. a. Fuses quietly. Schorlomite, Ca 3 (Fe.Ti.Al) 2 [(Si.Ti)0 4 ]3; Black; H. 7- 7.5; G. 3.88; F. 4; I. b. Fuses with intumescence. Tscheffkinite, Si, Ti, Th, Ce, Fe, Ca, O?; Velvet-black; H. 5-5.5 ; G. 4.55 ; F. 4 ; Massive. Rinkite, Na2, Can, (Ti.F 2 ) 4 (Si0 4 )i2 ; Straw-yellow, yel- lowish brown ; Cl. pinacoidal ; H. 5 ; G. 3.46 ; V. 2. Do not gelatinize with HC1. a. Fusibility below 5. TITANITE, CaTiSi0 3 ; Gray, brown, green, yellow, black; Cl. prismatic ; H. 5-5.5; G. 3.5; F. 4; V; p. 503. Guarinite, CaTiSiO 5 ; Sulphur- to honey-yellow; Cl. pinacoidal ; H. 6 ; G. 3.5 ; F. 4 ; IV. 668 MINERALOGY Keilhauite, CaTiSi0 5 (Y.Al.Fe) 2 Si0 5 ; Brownish black; Cl. prismatic ; H. 6.6; G. 3.65 ; F. 4-4.5; V. Neptunite, (Na.K)(Fe.Mn)TiSi 4 Oi 2 ; Black; Cl. pris- matic; H. 5-6; G. 3.23; F. 3.5; V. Benitoite, BaTiSiO 5 ; Colorless, blue; H.6.5; G. 3.64 ; III. Mosandrite, H^Na*, Ce 2 , Ca 10 [(Ti.Zr)(OH.F) 2 ]4(Si0 4 )i 2 ; Reddish to greenish brown; Resinous; Cl. one; H. 4 ; G. 2.96 ; Compact. b. Fusibility above 5. PEROVSKITE, CaTi0 3 ; Yellow, orange, brown, black ; Cl. cubic ; H. 5.5 ; G. 4.93 ; I ; p. 347. RUTILE, Ti0 2 ; Yellow, reddish brown to black ; Cl. prismatic ; H. 6-6.5 ; G. 4.22 ; Int. ; II ; p. 349. OCTAHEDRITE, TiO 2 ; Yellow, brown, blue, black; Cl. basal and pyramidic ; H. 5.5-6 ; G. 3.88 ; II ; p. 351. BROOKITE, Ti0 2 ; Hair-brown to black; H. 6; G. 3.94; IV; p. 351. Zirkelite, (Ca.Fe.UO 2 ) (Zn.Ti) 2 5 ; Black ; Resinous ; H. 5; G. 4.71; I. XII. The solution is blue (tungsten) or (columbium). Fusibility above 5. 1. The solution first assumes a violet color and then becomes blue. ^schynite, Cb, Ti, Th, Ce, La, Ca, Fe, 0?; Brownish black to black ; Resinous ; H. 6.5 ; G. 4.93 ; Inf. ; IV. Euxenite, Cb, Ti, Y, Er, Ce, U, Fe, H, O, ? ; Brownish black to black ; Resinous ; H. 6 ; G. 5.00 ; IV. Polycrase, Cb, Ti, Y, Er, Ce, U, Fe, H, O, ? ; Brownish black to black ; H. 6 ; G. 5.00. Pyrochlore, Cb, Ti, Ca, Na, O, Th, Ce, Fe, F?; Brown- ish black to black ; H. 5-5.5 ; G. 4.28 ; I. 2. The solution is blue without passing through violet. SCHEELITE, CaW0 4 ; White, yellow, green, brown; Cl. pyramidal ; H. 4.5-5 ; G. 6.05 ; II ; p. 543. Tungstite, WO 3 ; Yellow, greenish yellow; dull; IV; Earthy. Hatchettolite, Cb, Ta, U, Ca, O, H, Fe ? ; Yellowish brown ; Resinous ; H. 5 ; G. 4.85 ; I. TITANIUM AND TUNGSTEN MINERALS 669 Microlite, Ta, Cb, Ca, Na, C, F, H ? ; Pale yellow to brown ; Resinous ; H. 5.5 ; G. 5.5 ; I. FERGUSONITE, (Y.Er.Ce)(Cb.Ta)0 4 ; Brownish black; Resinous; H. 5.5-6; G. 5.80; II; Mas- sive. Sipylite, (Er.Ce.La.DLH 3 )CbO 4 , ? ; Brownish black; Resinous ; H. 6 ; G. 4.9 ; II ; Massive. Compare, Columbite p. 634. Wohlerite, Si, Zr, Cb, Ca, Na, O, ? ; Straw to brownish yellow; Cl . pinacoidal ; H. 5.5-6 ; G. 3.44; F. 3-3.5; V. XIII. The mineral in powder is dissolved in the S. Ph. bead. A. The S. Ph. bead in R. F. is yellow (nickel). 1. Effervesces in hot dilute HC1 (carbonates). Zaratite, (Ni.OH) 2 CO 3 .Ni(OH) 2 .4 H 2 O ; Emerald-green; H. 3 ; G. 2.63 ; F. Inf. ; Massive. 2. Do not effervesce with HC1. GENTHITE, H 4 Ni 2 Mg 2 (Si0 4 )3.4 H 2 ; Pale to deep- green ; Dull ; H. 3-4 ; G. 2.41 ; Inf. ; Amorph. ; p. 500. Morenosite, NiSO 4 .7 H 2 O ; Apple-green, greenish white ; Cl. pinacoidal ; H. 2 ; G. 2.00 ; IV. B. The S. Ph. bead is blue in both flames (cobalt). 1. Effervesces with HC1. Sphaerocobaltite, CoC0 3 ; Rose-red; H. 4; G. 4.07; Inf.; III. Remingtonite, CoC0 3 , + H 2 O ; Rose-red ; Soft ; Earthy ; Yields water. C. The S. Ph. bead is green in R. F. when cold (chromium or uranium) . 1. Shows chromium; ft, p. 569. UVAROVITE, Ca 3 Cr 2 (SiO 4 )3; Emerald-green; H. 7.5; G.3.42; Inf.; I; p. 445. Fuchsite, H 2 K(Al.Cr) 3 (SiO 4 ) 3 ; Emerald-green; Cl. basal ; H. 2.5 ; G. 2.86 ; F. 5 ; V ; Micaceous. Kammererite, H 8 Mg5(Al.Cr) 2 Si 3 Oi 8 ; Garnet to peach- blossom-red ; Cl. micaceous ; H. 2-2.5 ; G. 2.75 ; F. 5;V. 2. Shows uranium ; b, p. 576. Uranophane, CaU 2 Si 2 On.5 H 2 O ; Honey-, lemon-, or straw-yellow ; H. 2-3 ; G. 3.86 ; Inf. ; VI. Uranophilite,CaU 8 S 2 O 3 i.25H 2 0; Yellow; G. 3.75; Vel- vety, incrusted ; Inf. 670 MINERALOGY a. Yields a reaction for vanadium ( 6, p. 576), as well as for uranium. Carnotite, K 2 (UO 2 ) 2 (VO 4 ) 2 .3 H 2 O ; Yellow; Dull; H. 2.5; F. 3; Earthy. 3. Shows vanadium ; b, p. 576. Roscoelite, H 8 K 2 (Mg.Fe)(Al.V)4(SiO 3 )i2?; Clove-brown to brown-green; Pearly; Cl. basal; H. 2?; G. 2.93 ; F. 3. 4. Shows molybdenum ; 6, p. 586. a. The concentrated HC1 solution yields a white precipi- tate with H 2 SO 4 . Powellite, CaMoO 4 ; Colorless, green, yellow ; Resin- ous; H. 3.5; G. 4.52; F. 4; II. b. Yields no precipitate with H 2 S04. Belonesite, MgMoO 4 ; White ; F. 4-5 ; II. XIV. Minerals not included in the preceding groups. They are classified according to their fusibility, solubility in acids, and hardness. A. Fusibility below 5. 1. Hardness below 5. -f. Yields water, a. Soluble in HC1 or decomposed with the separation of silica. . Fuses with intumescence or exfoliates. VERMICULITE, Si, Al, Mg, O, (H 2 O), ?; Yellow, brown, light to dark green ; Pearly ; Cl. basal ; H. 1.5 ; G. 4-4.5 ; F. 4.5. The dilute HC1 solution yields a precipitate with H 2 S0 4 (barium). HARMOTOME, (Ba.K 2 )Al 2 Si 5 14 .5 H 2 O ; White; Cl. pinacoidal; H. 4.5.; G. 2.47; F. 3 ; V; p. 482. Brewsterite, H 4 (Sr.Ba.Ca)Al 2 (SiO 3 ) 6 .3 H 2 ; White, yellow, gray ; CJ. pinacoidal ; H. 5 ; G. 2.45 ; F. 3 ; V. Wellsite, (Ca.K 2 .Ba) Al 2 Si 3 O 10 .3 H 2 O ; White; H. 4-4.5; G. 2.32 ; F. 3 ; V. The dilute HC1 solution yields no precipitate with H 2 S0 4 . HEULANDITE, H 4 (Ca.Na 2 )Al 2 (SiO 3 ) 6 .3 H 2 ; White, yellow, red; Pearly, Cl. pinacoidal; H. 3.5-4; G. 2.20; F. 3; V; p. 481. STILBITE, H 4 (Ca.Na2) Al 2 (SiO 3 ) 6 .4 H 2 O ; White, yel- FUSIBILITY AND HARDNESS BELOW 5 671 low, brown, red ; Pearly; Cl. pinacoidal ; H. 3.5-4; G. 2.36; F. 3; V; p. 483. Gmelinite, (Na2.Ca)Al 2 (Si0 8 ) 4 .6 H 2 O ; White, yellow, flesh-red; Cl. prismatic; H. 4.5; G. 2.10; F. 3; III ; Rhombohedral. Epistilbite, H 4 (Ca.Na2)Al2(Si03)63 H 2 ; White ; Cl. pinacoidal ; H. 4-4.5 ; G. 2.25 ; V. ft. Fuses quietly. DEWEYLITE, H 4 Mg 4 (Si0 4 )3.4 H 2 O ; Yellow, brown, apple-green, resinous; H. 2.5-4; G. 2.40; F. 4-5; Amorphous. b. Gelatinizes in HC1. a. Shows calcium ; 6, p. 566. Gyrolite, H 2 Ca 2 (Si0 3 )3.H 2 ; White ; H. 3.4 ; F. 3 ; Radiated. Okenite, H 2 Ca(Si03) 2 .H 2 O ; White, cream, bluish white ; Dull ; H. 4.5-5 ; G. 2.28 ; F. 2.5-3 ; Fibrous, compact. ft. Becomes blue with cobalt solution. LAUMONTITE, (Ca, (A1.2 OH) 2 (Si 2 5 )2 H 2 ; White to gray; H. 3.5-4; G. 2.30; F. 2.5; V; p. 479. Gismondite, (Ca.K 2 )Al 2 (SiO 3 ) 4 .4H 2 O; White; H.4. 5; G. 2.26 ; F. 3 ; V. Levynite, CaAl(A1.2OH)(Si0 3 ) 3 .4H 2 ; White, gray red; H. 4.5; G. 2.13; F. 2.5; III. y. The dilute HC1 solution yields a precipitate with H 2 SO 4 (barium). Edingtonite, BaAl(A1.2 OH) (SiO 3 ) 3 .2 H 2 ; White, pink; Cl. prismatic; H. 4.5-5; G. 2,77; F. 2.5; IV. 8. Shows magnesium with cobalt solution ; p. 567. Spadaite, H 2 Mg 5 (Si0 3 )6.3 H 2 O ; Flesh-red; Pearly; H. 2.5 ; F. 4 ; Massive. c. Not soluble or decomposed in HC1. a. Yields a red flame with lithium flux (lithium), LEPIDOLITE, LiKAl(OH.F) 2 Al(Si0 3 ) 3 ; Lilac, grayish white ; Pearly ; Cl. Basal ; H. 2.5-4 ; G. 2.85 ; 2 ; V ; p. 495. Cookeite, Li(A1.2 OH) 3 (Si0 3 ) 2 ; White; Pearly; Cl. basal ; H. 2.5 ; G. 2.65 ; F. 4 ; V. ft. Decomposed with hot concentrated H 2 S0 4 ; all mica- ceous. *. Shows potassium. 672 MINERALOGY BIOTITE, (K.H) 2 (Mg.Fe) 2 (Al.Fe 2 ) 2 (SiO 4 )3; Green, yel- low, black; Cl. basal; H. 2.5; G. 2.90; F. 5; V; p. 492. PHLOGOPITE, (H.K) 8 (Mg.Fe)8(ALFe)(SiO 4 )3 ; Yel- lowish brown, green, white; H. 2.5-3; G. 2.86; F. 4.5-5; V; p. 494. **. Shows no potassium. CLINOCHLORE, H 8 Mg5Al 2 Si 3 Oi8 ; Green of various shades; H. 2.2; G. 2.72; F. 5; V; p. 497. y. Not decomposed with H 2 SO 4 . *. Yields a potassium flame through the blue glass ; Micaceous. MUSCOVITE, H 2 KAl 3 (SiO 4 )3 ; White, brown, green, yellow; H. 2-2.5 ; G. 2.86 ; F. 4.5-5; V; p. 489. Alurgite, H(K.Mg.OH) 2 (A1.0H)Al(SiO 3 )4; Rose-red to deep red ; Pearly ; H. 3 ; V. Yields water ; Not micaceous. HARMOTOME, (Ba.K 2 ) Al 2 Si 5 O 14 .5 H 2 O ; White; Cl. pinacoidai ; H. 4.5 ; G. 2.47 ; F. 3 ; V ; p. 482. Mordenite, (K 2 .Na2.Ca)Al 2 SiioO 2 4.6f H 2 ; White, yellow, pink; Cl. pinacoidai ; H. 3-4; G. 2.15; F. 4-5; V. **. Shows no potassium. Paragonite, H 2 NaAl3(SiO 4 )3 ; Yellow to grayish white; Pearly ; H. 2.5-3 ; G. 2.89 ; F. 5 ; V. MARGARITE, H 2 CaAl 4 Si 2 Oi 2 ; Pink, gray, white; Pearly; H. 3.5-4; G. 3.05; F. 4-4.5; V; p. 496. . Yields no water. Leucophanite, Na(BeF)Ca(SiO 3 ) 2 ; Pale green, yellow, white ; Cl. basal ; H. 4 ; G. 2.96 ; F. 2.5-3 ; IV. 2. Hardness above 5. +. Yields water in the closed tube, some only upon intense ignition, a. Decomposed or soluble in HC1. <*. Fuses with swelling or -intumescence. CHABAZITE, (Ca.Na2)Al 2 (SiO 3 ) 4 .6 H 2 O ; White, yel- low, red ; Cl. rhombohedral ; H. 5; G. 2.12; F. 3; III ; p. 484. APOPHYLLITE, H 7 KCa 4 (SiO 3 )8.4i H 2 O ; White, yel- low, rose, pale green ; H. 5 ; Cl. basal ; Pearly ; G. 2.35; F. 2; II. p. 480. FUSIBILITY BELOW, HARDNESS ABOVE 5 673 Brewsterite, H 4 (Sr.Ba.Ca)Al 2 (SiO s ) 6 .3 H 2 ; White, yellow, red ; Cl. pinacoidal ; H. 5 ; G. 2.45 ; F. 3 ; V ; Shows barium. Phillipsite, (Ca.K 2 .Na 2 )Al 2 Si 4 Oi 2 .4H 2 0; White; Cl. pinacoidal ; H. 5 ; G. 2.20 ; F. 3 ; V. Faujasite, H 2 (Ca.Na 2 )Al 2 (SiO 3 )5.9 H 2 O ; White, brown; Cl. octahedral; H. 5; G. 1.92; F. 3; I. (3. Fuses quietly. PECTOLITE, HNaCa^SiO-Og ; White, gray ; Cl. pina- coidal; H. 5; G. 2.73; F. 2.5-3; V; p. 427. ANALCITE, NaAl(SiO 3 ) 2 .H 2 O ; White; H. 5-5.5; G. 2.27 ; F. 3.51 ;- p. 485. Catapleiite, H 4 (Na 2 .Ca)ZrSi 3 On ; Yellow, brown, gray, violet ; Cl. prismatic ; H. 6 ; G. 6.28 ; F. 2.5. ; III ; Shows Zr. ; a, p. 571. b. Gelatinizes with HC1. a. Effervesces with HC1. CANCRINITE, H 6 (Na2.Ca)4(Al.NaC03)2Al 6 (Si0 4 )9 ; Yellow, pink, gray, white ; Cl. prismatic ; H. 5-6 ; G. 2.45; F. 2.5; III;, p. 441. Cenosite, Si, Y, Ca, O, CO 3 .H 2 O, ?; Yellowish brown; Greasy ; Cl. pinacoidal ; H. 5-5.5 ; G. 3.41 ; IV. /?. The HC1 solution yields a precipitate showing the presence of aluminium, but not calcium, b, p. 568. NATROLITE, Na 2 Al 3 (AlO)(Si0 3 )3.2H 2 O ; White ;'C1. prismatic; H. 5-5.5 ; G. 2.25; F. 2.5; IV; p. 486. Hydronephelite, HNa2Al(Si0 4 ) 3 .3 H 2 O ; White to dark gray; H. 5-6; G. 2.30; F. 2.5; III. y. Shows both aluminium and calcium. SCOLECITE, CaAl(A1.2 HO)(SiO 3 ) 3 .2 H 2 O ; White ; Cl. prismatic; H. 5-5.5; G. 2.30; F. 2.5; V; p. 479. MESOLITE, Na 2 Ca 2 Al 6 Si 9 3 o, 8 H 2 O ; White, gray, yellow; Cl. prismatic; H. 5 ; G. 2.29; F. 2.5; V; p. 478. THOMSONITE, (Ca.Na2)Al 2 (SiO 4 ) 2 .2i H 2 O ; White, gray; Cl. pinacoidal; H. 5-5.5; G. 2.35; F. 2.5; IV; p. 487. 8. Shows calcium but no aluminium; 6, p. 566. Okenite, H 2 Ca(Si0 3 ) 2 .H 2 O ; White, cream, bluish white ; Dull ; H. 4.5-5 ; F. 3 ; Fibrous, compact. 2x 674 MINERALOGY c. Insoluble or not decomposed in HC1. a. In the forceps yields a yellow flame (sodium). Epididymite, HNaBeSi 3 O 8 ; White; Cl. basal; H. 6; G. 2.55 ; F. 2.5-3 ; IV. Eudidymite, HNaBeSi 3 O 8 ; White; Cl. basal; H. 6; G. 2.55 ; F. 2.5-3 ; V ; Tabular. /?. Does not yield a strong sodium flame. EPIDOTE, Ca(Al.OH)(Al.Fc)i(SiO4)ij Yellowish to blackish green, gray; Cl. basal; H. 6-7; G. 3.40; F. 3-4 ; V ; p. 466. ZOISITE, Ca2(Al.OH)Al 2 (Si0 4 ) 3 ; Grayish white, pink, green; Cl. pinacoidal ; H. 6-6.5; G. 3.26; F. 3-4; IV; p. 465. Clinozoisite, Ca^Al.OH) Al 2 (SiO 4 ) 3 ; White to pale pink ; Cl. basal ; H. 6-7 ; G. 3.37 ; F. 3-4 ; V. Lawsonite, Ca(A1.2 OH)(SiO 4 ) 3 ; Grayish blue to white ; Cl. pinacoidal ; H. 8 ; G. 3.09 ; F. 4 ; IV. . Yields no water in the closed tube. a. Decomposed in HCi with separation of silica, but with- out gelatinizing. LABRADORITE, CaAlSi 3 8 ; White, gray; Cl. basal and pinacoidal ; H. 6 ; G. 2.73 ; F. 4 ; VI ; p. 416. WERNERITE, Ca 4 Al 6 , Si 6 ,O 2 5.Na 4 Al 3 Si 9 24 Cl. ; White, gray, light green ; Cl. prismatic ; H. 5.6 ; G. 2.68 ; F. 3 ; II ; p. 453. b. Gelatinizes with HCI. . Yields a sulphur reaction with soda and a sodium flame in the forceps. LAZURITE, (Na4.Ca) 2 (Al.Na.Ss)Al2(Si0 4 )s ; Deep azure to greenish blue; H. 5-5.5; G. 2.42; F. 3.5; I; p. 438. NOSELITE, Na 4 (Al.NaSO 4 ) Al 2 (SiO 4 ) 3 ; Gray, green, blue, brown ; H. 5.5 ; G. 2.32 ; F. 3.5-4 ; I ; p. 438. HAUYNITE, (Ca.Nas) 2 (Al.NaSO 4 )Al 2 (SiO 4 ) 3 ; Blue, green, yellow, white; H. 5.5-6; G. 2.45; F. 4; I; p. 438. Microsommite, Si, Al, Ca, Na, K, 0, Cl, (SO 4 ), (CO,),?; White; Cl. prismatic; H. 6 ; G. 2.44; F. 3.5; III. . Shows aluminium, &, p. 568. *. Shows chlorine ; a, p. 588. Yields a sodium flame in the forceps. FUSIBILITY BELOW, HARDNESS ABOVE 5 675 SODALITE, Na 4 (AlCl) Al 2 (Si0 4 ) 3 ; White, gray, blue, green; H. 5.5-6; G. 2.30; F. 3.5; I; p. 438. **. Yields a strong sodium flame, but no chlorine. NEPHELITE, (Na 2 .K 2 .Ca) 4 Al 8 Si 9 3 4; White, gray, greenish, reddish ; Cl. prismatic ; H. 5.5-6 ; G. 2.60 ; F. 4 ; III ; p. 440. ***. Yields calcium ; 6, p. 566. ANORTHITE, CaAl 2 (SiO 4 ) 2 ; White, gray; Cl. basal and pinacoidal; H. 6-6.5; G. 2.75; F. 4.5; VI; p. 413. MELILITE, Si, Al, Fe, Ca, Mg, Na, 0? ; Green, yellow, brown, white ; Cl. basal ; H. 5 ; G. 3.00 ; F. 4 ; II ; p. 451. Sarcolite, (Ca.Na^AlsCSiC^)-? ; Flesh to rose-red, white ; H. 6 ; G. 2.7 ; F. 3 ; II. y. Contains no aluminium. *. Shows calcium, 6, p. 566. WOLLASTONITE, CaSIO 3 ; White, gray; Cl. pina- coidal; H. 5-5.5; G. 2.85; F. 4; V; p. 429. **. Shows zirconium; a, p. 571. Hiortdahlite, (Na2.Ca)(Si.Zr)O 3 ? ; Straw-yellow to yel- lowish brown; H. 5.5-6; G. 3.26; F. 3; VI. c. Insoluble in HC1. . Yields a red flame in the forceps (lithium). SPODUMENE, (Li.Na)Al(SiO 3 ) 2 ; White, gray, pink, green; Cl. prismatic; H. 65-7; G. 3.18; F. 3.5; V; p. 426. PETALITE, (Li.Na)Al(Si 2 O 5 ) 2 ; White, gray, pink; Cl. basal; H. 6-6.5; G. 2.40; F. 4; V; p. 426. /8. Yields a potassium flame through the blue glass when fused with potassium flux ; a, p. 563. ORTHOCLASE, KAlSi 3 8 ; White, gray, cream, flesh- red, green ; Cl, basal and pinacoidal ; H. 6 ; G. 2.57 ; F. 5 ; V ; p. 403. MICROCLINE, Same, but VI, p. 409. Yields a little water in the closed tube at a high tem- perature. Milarite, HKCa 2 Al 2 (Si 2 O 5 ) 6 ; White to pale green; H. 5.5-6 ; G. 2.55 ; F. 3 ; III y. Yields a strong sodium flame in the forceps. 676 MINERALOGY ALBITE, NaAlSi 3 O 8 ; White to gray ; Cl. basal and pinacoidal; H. 6 ; G. 2.62; F. 4; VI; p. 411. See p. 403 for intermediate feldspars which may be of various colors. JADEITE, NaAlSi 2 O 6 ; White, grayish green; Cl. prismatic; H. 7; G. 3.33; F. 2.5; V; p. 426. Glaucophane, Na2Al 2 (SiO 3 ) 4 .Mg 4 (SiO3) 4 ; Lavender to azure-blue ; Cl. prismatic ; H. 6-6.5 ; G. 3.11 ; F. 3-4. Epididymite, HNaBeSi 3 O 8 ; White; Cl. basal; H. 6; G. 2.55; F. 2.5-3; IV. Eudidymite, HNaBeSi 3 O 8 ; White ; Pearly ; Cl. basal ; H. 6 ; G. 2.55 ; F. 2.5-3 ; V ; Tabular. Shows chlorine, 587. WERNERITE, Ca4Al 6 Si6O25.Na4Al 2 Si 9 O 2 4Cl ; White, gray, light green; Cl. prismatic; 2.5-6; G. 2.68; F. 3; II; p. 453. Marialite, Na 4 Al 3 Si9O24Cl; White; H. 5.5-6; G. 2.56; F. 3-4; II. 8. Yields test for aluminium and calcium; 6, p. 566. *. Fuses quietly. ANORTHITE, CaAl 2 (SiO 4 ) 2 ; White to gray; Cl. basal and pinacoidal ; H. 6 ; G. 2.75 ; F. 4.5 ; VI ; p. 413. GROSSULARITE, Ca 3 Al 2 (SiO 4 )3 ; Pale red, yellow, green, white; H. 6.5-7.5; G. 3.50; F. 3 ; I ; p. 443. HORNBLENDE, Complex silicate; Green to black ; CL prismatic; H. 6; G. 3.15; F. 3-4; V; p. 431. AUGITE, Complex silicate; Greenish black to black; Cl. prismatic ; H. 57; G. 3.30; F. 4-45; V; p. 418. **. Fuses with intumescence. PREHNITE, H 2 Ca2Al 2 (SiO 4 ) 3 ; Apple-green, gray, white ; H. 6-6.5; G. 2.9; F. 2.5; IV; p. 470. VESUVIANITE, Complex silicate; H. 6.5; G. 3.40; F. 3; II; p. 455. EPIDOTE, Ca 2 (AlOH)(Al.Fe) 2 (SiO 4 ) 3 ; Yellowish to blackish green, gray ; Cl. basal ; H. 6-7 ; p. 466. ZOISITE, Ca2(AlOH)Al 2 (SiO 4 )3; Grayish white, green, pink; Cl. pinacoidal; H. 6-6.5; G. 3.26; F. 3-4 ; V; p. 465. Clinozoisite, Ca2(AlOH) 2 (Al)(SiO 4 )3; White to pale pink; Cl. basal; H. 6-7 ; G. 3.37; F. 3-4; V; p. 465. FUSIBILITY ABOVE, HARDNESS BELOW 5 677 e. Shows aluminium and magnesium, but little or no calcium; b, p. 567. PYROPE, Mg s A] 2 (SiO 4 ) 3 ; deep red ; H. 6.5-7.5 ; G. 3.72 ; F. 3.5-4; I; p. 443. . Shows calcium and magnesium, but little or no alu- minium ; 6, p. 567. TREMOLITE, CaMg 3 (SiO 3 ) 4 ; White, gray, violet; Cl. prismatic; H. 5-6 ; G. 3.00; F. 4; V; p. 431. DIOPSIDE, CaMg(SiO 8 ) 2 ; White to pale green; H. 5-6; G. 3.29; F. 4; V; p. 423. PYROXENE, Ca(Mg.Fe)(Si0 3 ) 2 ; Light to dark green; Cl. prismatic; H. 5-6; G. 3.3; F. 4; V; p. 419. ACTINOLITE, Ca(Mg.Fe) 3 (SiO 3 ) 4 ; Various shades of green; CL prismatic; H. 5-6; G. 3.10; F. 4; V; p. 431. y. Shows magnesium and iron, but little or no calcium, 6, p. 567. ANTHOPHYLLITE, (Mg.Fe)SiO 3 ; Gray, clove-brown to green; Cl.. prismatic; H. 5.5-6; G. 3.10; IV. ENSTATITE, (Mg.Fe)SiO 3 ; Gray-brown to green; Pearly, bronze-like; Cl. pinacoidal ; H. 5.5-6.5; G. 3.20; F. 5-6; IV; p. 421. 0. Shows magnesium, but no silica. Reacts for fluorine ; a, p. 589. Sellaite, MgF 2 ; White; Cl, basal; H. 5; G. 3.06; F. 4-5; II. B. Fusibility above 5. 1. Hardness below 5. + . Yields water, a. Soluble in HC1, or decomposed with the separation of silica. a. Yields a sulphur reaction with soda. ALUMINITE, A1 2 (OH) 4 SO 4 .7 H 2 O ; White ; Dull ; H. 1-2; G. 1.66; V; p. 542. Felsobanyite, A1 2 (OH) 4 SO 4 .2 A1(OH) 8 .5 H 2 O ; White to pearly; Cl. perfect; H. 1 5 ; G. 2.33 ; III; Scaly. Alumian, (A1 2 O)(SO 4 ) 2 ; White; H. 2-3; G. 2.7; III; Massive ; Yields no water. Yields a violet flame (potassium). Lowigite, K(A1.2 OH) 3 (SO 4 ) 2 .li H 2 O ; Straw-yellow ; H. 3-4 ; G. 2.58 ; Massive. 678 MINERALOGY ft. Yields a fluorine reaction. Fluocerite, (Ca.La.Di) 2 OF 4 .OH ; Reddish yellow ; Cl. two directions; H. 4; G. 5.80; Massive; III. Yttrocerite, (Y.Er.Ce)F 3 .5 CaF 2 .H 2 ; Violet, gray, brown, white; Greasy; H. 2.5-3; G. 3.45; Mas- sive. y. Ignited with cobalt solution becomes flesh-pink (magnesium). DEWEYLITE, H 4 Mg 4 (Si0 4 )3.4 H 2 O ; Yellow to apple- green ; Resinous ; " H. 3-4 ; G. 2.40 ; Amorphous. Sepiolite, H 4 Mg 2 Si3O 10 ; White to grayish white ; Dull ; H. 2-2.5; G. 2.0; Compact. Usually too dark in color to show pink with cobalt solution. SERPENTINE, H 4 (Mg.Fe) 3 Si 2 09 ; Olive, blackish, or yellowish green, white; Greasy; H. 2.5-6, G. 2.56; Massive; p. 498. 6. Gelatinizes with HC1. ALLOPHANE, Al 2 Si0 5 .H 2 O; White, yellow, green, blue; H. 3; G. 1.88; Amorphous, p. 502. c. Insoluble in HC1. a. Micaceous. CLINOCHLORE, H 8 Mg 5 Al 2 Si 3 Oi8 ; Green of various shades; H. 2-2.5; G. 2.75; V; p. 497. Clintonite, H3(Mg.Ca) 5 Al 5 Si 2 O8; Reddish brown, cop- per-red; Pearly; H. 4-5 ; G. 2.87; V. Xanthophyllite, H 8 (Mg.Ca)i4Ali 6 Si 5 O5 2 ; Light green ; H. 4-5; G. 3.09; V. Prochlorite, H 4 o(Fe.Mg) 2 3Al 14 Sii 2 09o?; Green to black- ish green; H. 1-2; G. 2.87; V. j8. Becomes blue with cobalt solution, PYROPHYLLITE, H 2 Al 2 (Si0 3 ) 4 ; White, apple-green, gray, brown; Pearly; H. 1-2; G. 2.85; Foliated, compact; p. 502. KAOLINITE, H 4 Al 2 Si 2 9 ; White; Pearly, dull; Cl. basal; H. 2-2.5; G. 2.60; V; p. 501. GIBBSITE, AI(OH) 3 ; White; Pearly, dull ; Cl. basal; H, 2.5-3.5; G. 2.35; V; p. 365. BAUXITE, A1 2 O(OH) 4 ; White, gray, yellow, red; Dull, earthy ; G. 2.55 ; Massive, clay-like ; p. 366. y. Yields a sulphur reaction when fused with soda. FUSIBILITY ABOVE, HARDNESS BELOW 5 679 ALUNITE, (K.Na) (A1.2 OH) 3 (SO 4 ) 2 ; White, gray ; Cl. basal; H. 3.5-4; G. 2.85 ; III; p. 541. 8. With potassium bisulphate yields a fluorine reac- tion; a, p. 589. RalstoAite, (Na 2 .Mg)F 2 .3 A1(F.OH) 8 .2 H 2 ; White to straw-yellow; H. 4.5; G. 2.59; I. Prosopite, Ca(F.OH) 2 .2Al(F.OH) 3 ; White, gray; H. 4.5; G. 2.89; V. Fluellite, A1F 3 .H 2 O ; White; H. 3 ; G. 2.17; IV. e. With cobalt solution becomes flesh-pink (magnesium) . TALC, H 2 Mg 3 (SiO 3 )4; Apple-green, gray, white; Pearly, greasy ; Cl. basal ; H. 1 ; G. 2.80 ; Foliated, com- pact; p. 500. . Yields no water. a. Fused with potassium bisulphate yields a fluorine re- action. Tysonite, (Ce.La.Di)F 3 ; Wax-yellow, reddish brown; Cl. basal; .H. 4.5-5; G. 6.13; III. Bastnasite, (Ce.La.Di.F)CO 3 ; Wax-yellow, reddish brown; H. 4.5-5; G. 5.08; Massive. Hardness above 5. + . Yields water, a. Soluble in HC1. Pollucite, H 2 Cs 4 Al4(Si03)9 ; White; H. 5.5-6 ; G. 2.98; I ; Massive. 6. Gelatinizes with HC1. a. Fused with potassium bisulphate shows fluorine. CHONDRODITE, Mg 3 [Mg(F.OH)] 2 (Si0 4 ) 2 ; Brownish red, yellow, white ; Cl. basal ; H. 6-6.5 ; G. 3.15 ; V ; p. 471. Humite, Mg 5 [Mg(F.OH)] 2 (SiO 4 ) 3 ; Brownish red, yel- low, white; Cl. basal; H. 6-6.5; G. 3.15; IV. Clinohumite, Mg 7 [Mg(F.OH)] 2 (SiO 4 )4; Brownish red, yellow, white; Cl. Basal; H. 6-6.5; G. 3.15; V. Prolectite, Mg[Mg(F.OH)] 2 SiO 4 ; Brownish gray; V. 13. Yields the rare earths ; p. 572. CERITE, (Ca.Fe) (CeO) (Ce 2 .3 OH) (Si0 3 ) 3 ; Clove- brown, gray, red; Dull, resinous; H. 5.5; G. 4.90; IV. Thorite, ThSiO 4 , Orange-yellow, brown, black; Resi- nous ; Cl. prismatic ; H. 5 ; G. 4.90 ; II. 680 MINERALOGY c. Insoluble in HC1 or not decomposed. a. The finely powdered mineral when fused with one and one half parts of soda yields a bead remaining clear when cold. OPAL, SiO 2 with water; White, colors of various shades; H. 5.5-6.5; G. 2.15; Amorphous; p. 369. p. Becomes blue with cobalt solution. DIASPORE, AIO(OH) ; White, gray, yellowish, green- ish; Cl. pinacoidal; H. 6.5-7; G. 3.40, IV; p. 365 Zunyite, [A1.2 (OH.F.Cl)] 6 Al 2 (Si.0 4 )3; White, gray; H. 7; G. 2.88; I. y. May yield only a very small amount of water. STAUROLITE, (A10) 4 (Al.OH)Fe(SO 4 ) 2 ; Red-brown to brownish black; Cl. pinacoidal; H. 7-7.5; G. 3.65; IV; p. 477. IOLITE, H 2 (Mg.Fe)4Al 8 Siio037 ; Light or dark blue, white; Cl. pinacoidal ; H. 7-7.5; G. 2.61 ; IV; p. 440. 8. Shows berlium ; p 571. Lavender with cobalt solu- tion. Bertrandite, Be 2 (Be.OH) 2 Si 2 7 ; White, yellow ; Pearly ; Cl. prismatic; H. 6-7; G. 2.60; IV. Euclase, Be(A1.0H)SiO 4 ; White to pale green; Pearly; Cl. pinacoidal; H. 7.5; G. 3.10; V. c. Fused with soda yields a sulphur reaction. Melanophlogite, SiO 2 SO 3 H 2 0,etc. ? ; White, light brown ; H. 6.5-7; G. 2.02; I; Cubes. 2. Hardness above 5. . Yields no water. a. Soluble or decomposed with HC1. a. Fused with potassium flux, shows potassium through the blue glass. LEUCITE, KAl(Si0 3 ) 2 ; White, gray; H. 5.5-6: G. 2.47; I; p. 416. b. Gelatinizes with HC1. CHRYSOLITE, (MgFe)Si0 4 ; Olive to grayish green, brown; Cl. pinacoidal; H. 6.5-7; G. 3.33; IV; p. 446. . Contains no iron. Forsterite, Mg 2 SiO 4 ; White, gray, yellowish ; Cl. pina- coidal; H. 6.5-7; G. 3.24; IV; p. 450. FUSIBILITY AND HARDNESS ABOVE 5 681 ft. The concentrated HC1 solution yields a precipitate with H 2 SO 4 (calcium). Gehlenite, (Ca.Mg.Fe) 3 Al 2 Si2Oi ; Grayish green, brown ; H. 5.5-6 ; G. 2.98 ; F. 5 ; II. y. Yields a test for the rare earths. GADOLINITE, FeB 2 Y 2 Si 2 Oio ; Black, greenish black, brown ; H. 6.5-7 ; G. 4.25 ; V. Thorite, ThSi0 4 ; Orange-yellow, brown, black; Resi- nous; Cl. prismatic; H. 5; G. 4.90; II. c. Insoluble in HC1. a. Yields a fine blue with cobalt solution. CORUNDUM, A1 2 O 3 ; All colors; Cl. rhombohedral ; H. 9; G. 4.03; III; p. 341. CYANITE, Al(AlO)Si0 4 ; Blue, gray, white, green; Cl. pinacoidal; H. 5-7 ; G. 3.62; VI; p. 461. ANDALUSITE, Al(AlO)SiO 4 ; Flesh-red, reddish brown, olive; Cl. prismatic; H. 7.5; G. 3.18; IV; p. 459. SILLIMANITE, Al(A10)SiO 4 ; Brown, gray, greenish gray; Cl. pinacoidal; H. 6-7; G. 3.23; IV; p. 461. Dumortierite, Al 2 (A10) 6 (Si0 4 ) 3 ; Deep blue; CL pina- coidal ; H. 7 ; G. 3.26 ; IV. TOPAZ, Al(Al(O.F 2 ))SiO 4 ; White, yellow, pink, bluish, greenish; Cl. basal; H. 8 ; G. 3.53 ; IV; p. 458. Kornerupine, Mg(A10) 2 Si0 4 ; White, yellowish brown; Cl. prismatic ; H. 6.5 ; G. 3.27 ; IV. CHRYSOBERYL, BeAl 2 O 4 ; Yellowish to emerald- green ; Cl. prismatic ; H. 8.5 ; G. 3.67 ; IV ; p. 377. y. Do not yield a fine blue with cobalt solution, some however may give a dull blue. *. Fused with 1| parts of soda yields a bead remain- ing clear when cold. QUARTZ, SiO 2 ; All colors; H. 7; G.2.65; III; p. 352. TfclDYMITE, SiO 2 ; White; H. 7; G. 2.30; III; p. 361. CHALCEDONY, SiO 2 ; Various colors; Waxlike; H. 7 ; G. 2.62 ; Massive ; p. 360. **. Do not yield a clear bead. Shows magnesium c, p. 567. ENSTATITE, (Mg.Fe)SiO 3 ; Gray, brown, green; Pearly, bronzelike; Cl. prismatic; H. 5.5-6.5; G, 3.20; IV; p 421. 682 MINERALOGY HYPERSTHENE (Mg.Fe)SiO 3 ; Brownish green to greenish black ; Pearly ; Cl. pinacoidal ; H. 5-6 ; G. 3.45; IV; p. 421. ANTHOPHYLLITE, (Mg.Fe)Si0 3 ; Gray, clove brown, green; Pearly; Cl. prismatic; H. 5.5-6; G. 3.10; IV. SPINEL, Mg(AlO) 2 ; Various colors ; H. 8 ; G. 3.5-4 ; I; p. 371. , Yields a zirconium reaction a, p. 571. ZIRCON, ZrSiO 4 ; Brown, white, gray, green, red; Cl. prismatic; H. 7.5; G. 4.68; II; p. 456. Baddeleyite, ZnO 2 ; White, yellow, brown, black; Cl. basal; H. 6.5; G. 5.5; V. Shows beryllium, a, p. 571. BERYL, Be 3 Al 2 (Si.O 3 ) 6 .iH 2 0; White, green, yellow, blue, pink; H. 7-7.5; G. 2.69; III; p. 436. PHENACITE, Be 2 SiO 4 ; White ; Cl. prismatic ; H. 7.5-8 ; G. 2.96 ; III ; p. 452. Powdered and reduced with soda yields magnetic particles. Hercynite, Fe(A10) 2 ; Black ; H. 7.5-8 ; G. 3.93 ; I. Extremely hard. DIAMOND, C ; Colorless, yellow, red, blue, gray, black ; Cl. tetrahedral; H. 10; G. 3.52; p. 281. INDEX Numbers printed in italics refer to Part I ; those in black-faced type, to Part II ; and those in roman type, to Part III. Abbreviations, 595, 617. Acanthite, 301, 630. Achroite, 475. Acicular habit, 266. Acid salts, 227. Acids, silicic, 233. Acmite, 420, 660. Actinolite, 433, 677. Acute bisectrix, 173. Adamantine luster, 279. Adamantine spar, 343. Adamite, 513. Adelite, 653. Adularia, 408. ^Enigmatite, 435, 659. ^Eschynite, 634, 668. Agate, 360. Moss, 360. Aggregate, 265. Agricolite, 651. Aguilarite, 627. Aikinite, 628. Airy's spirals. 197. Alabandite, 304, 631, 666. Alabaster, 537. Albite, 411, 604, 614, 676. Alexandrite, 378. Algodonite, 621. Alkalies, 222, 562. Alkaline earths, 565. Alkaline reaction, 565. Allactite, 652. Allanite, 468, 596, 604, 633, 658. Allemontite, 620. Alloclasite, 622. Allophane, 502, 678. Almandite, 444, 659. Altaite, 625. Alternating axis, 15. symmetry, 15. Alum, 542. Alumian, 677. Aluminite, 233, 337, 542, 677. Aluminium, Tests, 568. Alunite, 541, 679. Alunogen, 542, 600, 638. Alurgite, 672. Amalgam, 631. Amaranite, 636. Amazon stone, 411. Amblygonite, 512, 657. Amethyst, 359. Ammonium, tests for, 565. Amorphous, 3, 238. Amphiboles, 431, 432, 604, 608, 615. Amplitude, 181. Amygdaloidal, 271. Amygdule, 271. Analcite, 485, 603, 610, 673. Analogous pole, 475. Analyzer, 177. Analysis, 227. Anatase (octahedrite) , 347, 351. Andalusite, 459, 605, 616, 681. Andesine, 414. Andorite, 623. Andradite, 444, 659. Angle, between faces, 10. between normals, 12. between optic axes, 205. constancy of, 10. critical, 166. of extinction, 185. Anglesite, 532, 602, 647. Anhydrite, 531, 602, 662. Anisotropic, 163. Ankerite, 657. Annabergite, 316, 517, 651. Anomite, 493. Anorthite, 413, 604, 676. Anthophyllite, 677, 682. Antilogous pole, 475. Antimonides, 294. Antimony, 293, 623. glance, 295. oxides, 346. tests, 584. Apatite, 96, 508, 603, 611, 655. Aphthitalite, .637. Apjohnite, 639. Aplome, 444. Apophyllite, 480, 603, 672. 683 684 INDEX Apparatus blowpipe, 546. Apparent angle, 206. Aquamarine, 437. Aragonite, 392, 602, 660. Ardennite, 665. Arfvedsonite, 435, 659. Argentite, 397, 596, 630. Argyrodite, 630. Arsenates, 236. Arsenic, 393, 620. tests, 585. Arsenical pyrite, 318. Arsenic oxides, 346. Arsenides, 232, 294. Arseniosiderite, 652. Arsenolite, 346, 640. Arsenopyrite, 318, 597, 622. Arzrunite, 642. Asbestos, 433, 500. Asbolite, 369. Astrophyllite, 659. Assay, 578. Asymmetric, 132. Atacamite, 334, 644. Atomic weights, 223. Augelite, 656. Augite, 123, 424, 676. Aurichalcite, 641. Autunite, 520, 655. Awaruite, 632. Axes, Crystallographic, 15. calculations, 83, 128. hexagonal, 85. monoclinic, 128. orthorhombic, 120. projection of, 35. Axial angles, 16. optic,- 205. Axial planes, 16. Axial ratios, 24. Axinite, 469, 664. Axis, didigonal, 14. .- digonal, 14. dihexagonal, 14. ditetragonal, 14. hexagonal, 14. of alternating symmetry, 15. optic, 198. polar, 55. trigonal, 14. tetragonal, 14. twinning, 138. zonal, 22. Azurite, 399, 608, 641. Babingtonite, 660. Baddeleyite, 682. Bakerite, 664. Balance, Joly's, 261. Westphal, 264. Barite, 116, 528, 601, 662. Barium, tests, 565. Barkevikite, 435. Barrandite, 654. Barysilite, 648. Barytocalcite, 395, 660. Basal cleavage, 256. Basal pinacoid, 68. Basic salts, 226. Base, 68. Bastnasite, 679. Baumhauerite, 622. Bauxite, 366, 602, 678. Baveno twins, 406. Bayldonite, 643. Bechilite, 664. Beegerite, 628. Belonesite, 670. Bementite, 666. Benitoite, 103, 505, 668. Beresonite, 647. Berthierite, 320, 625. Bertrandite, 680. Beryl, 89, 436, 605, 682. Beryllium, tests, 571. Beryllonite, 657. Berzelianite, 627. Berzeliit'e, 652. Beudantite, 646. Bevelment, 22. Beyrichite, 307, 630. Biaxial, 171. Biaxial interference, 200. Bieberite, 637. Bindheimite, 647. Binnite, 621. Biotite, 492, 601, 615, 660, 672. Birefringency, 168. sign of, 194. Bisectrices, 173. dispersion, 207. Bismite, 346. Bismuth, 293, 629. tests, 580. Bismuthindte, 296, 629. Bismutite, 650. Bismutosmaltite, 622. Bismutosphserite, 650. Bixbyite, 633. Black jack, 301. Black sands, 288. Blende, 301. Bloedite, 539, 637. Bloodstone, 360. Blue earth, 283, 444. Blue vitriol, 540. Bobierrite, 657. 3og iron ore, 363. Boleite (percylite), 644. INDEX 685 Bone ash, 559. Bone phosphate, 509. Bone turquoise, 517. Boracite, 523, 663. Borates, 236, 507. Borax, 522, 600, 640. Borax bead, 553. Borickite, 654. Bornite, 310, 599, 629. Boron, tests, 591. Bort, 282. Botryogen, 636. Botryoidal, 273. Boulangerite, 321, 624. Bournonite, 321, 597, 623. Boussingaultite, 638. Brachy-axis, 113. Brachydome, 115. Brachypinacoid, 116. Brachyprism, 115. Brackenbuskite, 649. Brandtite, 652. Braunite, 632. Brazilian twins, 146. Breithauptite, 309, 625. Breunnerite, 657, 661. Brewsterite, 479, 670, 673. Brittle, 258. Brittle micas, 496. ' Brochantite, 541, 642. Bromine, tests, 588. Bromlite, 395, 660. Bromyrite, 330, 645. Brongniardite, 623. Bronzite, 421. Brookite, 351, 634, 668. Brucite, 91, 362, 601, 663. Brushite, 656. Bunsenite, 340. Bytownite, 414. Cabrerite, 651. Cacoxenite, 655. Cadmium, tests, 581. Caesium, tests, 564. Calamine, 118, 472, 603, 650. Calaverite, 626. Calcioferrite, 654. Calciovolborthite, 645. Calcite, 91, 380, 602, 612, 660. twins, 145, 146, 382. Calcium, tests, 566. Calculations of axes, 83, 112, 120, 128. formula, 231. 2 V, 205. Caledonite, 642. Callainite, 656. Calomel, 641. Cancrinite, 441, 673. Canfieldite, 630. Cape ruby, 443. Capillary, habit, 266. Cappelenite, 665. Caracolite, 646. Carat, 282. Carbon, tests, 590. Carbonado, 282. Carbonates, 233, 370. Carbonates, basic, 227. Carbuncle, 444. Carlsbad twins, 148, 405. Carminite, 646. Carnallite, 335, 639. Carnelian, 360. Carnotite, 514, 670. Carpholite, 666. Carposiderite, 637. Caryocerite, 665. Cassiterite, 347, 596, 598, 631, 646. Castanite, 636. Catapleiite, 673. Cat's-eye, 379. Celestite, 530, 662. Celsian, 414. Cenosite, 673. Center of symmetry, 16. Central, ditesseral, 47, tesseral, 59. Centrosymmetric, 130. Cerargyrite, 330, 601, 645. Cerite, 679. Cerium, tests, 572. Cerussite, 396, 602, 647. Cervantite, 296. Chabazite, 484, 603, 672. Chalcanthite, 540, 641. Chalcedony, 360, 681. Chalcocite, 300, 596, 629. Chalcomenite, 645. Chalcophanite, 632. Chalcophyllite, 643. Chalcopyrite, 310, 599, 629. Chalcosiderite, 643. Chalcostibite, 320, 623. Chalcotrichite, 339. Chalk, 385. Chalybite, 388. Charcoal, 551. coats on, 552. Chemical formula, 231. Chenevixite, 642. Chessylite, 399. Chiastolite, 460. Childrenite, 654. Chile saltpeter, 520. Chiolite, 662. Chiviatite, 628. Chloanthite, 315, 622. Chlor-apatite, 509. Chlorides, 327. 686 INDEX Chlorine, tests, 588. Chlorite, 497, 601, 615. Chloropal, 659, 660. Chlorophane, 332. Chondrodite, 471, 679. Chromates, 236. Chromic iron ore, 376. Chromite, 233, 337, 376, 596, 609, 659. Chromium, tests, 568. Chrysoberyl, 377, 681. Chrysocolla, 503, 601, 644. Chrysolite, 446, 680. Chrysoprase, 360. Chrysotile, 500. Churchite, 657. Cinnabar, 304, 598, 629, 641. Cinnamon stone, 443. Circular polarization, 196. Cirrolite, 655. Citrine, 359. Classes of crystals, 4?- Classification of minerals, 222, 232. Claudetite, 346, 640. Clausthalite, 627. Clay, 502. Cleavage, 256. Cleveite, 525. Cliftonite, 284. Clinoaxis, 120. Clinochlore, 497, 672, 678. Clinoclasite, 643. Clinodome, 123. Clinographic projections, 33. Clinohedrite, 127, 650. Clinohumite, 471, 679. Clinopinacoid, 123. Clinozoisite, 465, 674, 676. Clintonite, 496, 497, 678. Closed tube, 557. Clove oil, 215. Cobalt bloom, 517. Cobalt solution, 562. Cobalt, tests, 574. Cobaltite, 316, 597, 621. Colemanite, 524, 664. Collophanite, 655. Coloradoite, 626. Color, of beads, 594. of coats, 593. of flame, 552, of minerals, 273. order of, 183. Columbates, 236. Columbite, 505, 596, 634. Columbiun, tests, 570. Columnar habit, 265. Combinations of forms, 22, 53. Common garnet, 442. Common opal, 369. Common salt. 327. 634, Compact structure, 268. Compensation, 189. ' Compensator, 189. Composition, chemical, 231. plane, 138. Comptonite, 488. Conchoidal fracture, 258. Conichalcite, 643. Connellite, 642. Constancy of angles, 10. Constitutional formula, 234. Constitution, water of, 226. Contact goniometer, 12, 149. Contunnite, 641. Convergent light, 191. Cookeite, 496, 671. Copiapite, 636. Copper, 288, 598, 631. glance, 300. pyrite, 310. tests, 580. Copperas, 539. Coprolites, 5,09. Coquimbite, 540, 636. Cordierite, 440. Cornwallite, 643. Corundophilite, 498. Corundum, 91, 341, 605, 611, 681 Corynite, 622. Cosalite, 628. Cotunnite, 647. Covellite, 306, 629, 644. Crednerite, 631. Critical angle, 166. Crocidolite, 435, 659. Crocoite, 533, 649. Cronstedtite, 658. Crookesite, 627. Crossed dispersion, 209. Crossed nicols, 186. Cryolite, 333, 601, 662. Crystal, 6. axes, 15. aggregate, ^135. forms, 21. measurement, 153. structure, 4. Crystalline, 3, 238. 'rystalline elements, 24- Crystallization, 6, 245. Crystals, 1. optical properties, 160. physical properties, 256. 'ubanite, 629. !ube, 51. Cubic system, Jfl- "'ullinan diamond, 283. umengite, 644. Cuprite, 337, 598, 631, 644. : uprobismutite, 628. uproiodargyrite, 645. INDEX 687 Cuprotungstite, 544, 645. Curvature of face, 268. Curve of hardness, 259. Cyanite, 461, 604, 615, 681. Cyanochroite, 641. Cyanotrichite, 642. Cyclic twins, 142. Cylindrite, 624. D Danalite, 649. Danburite, 664. Darapskite, 637. Datolite, 463, 603, 664. Dawsonite, 661. Decrepitation, 552. ' Demantoid, 444. Dendritic, 269. Density, 260. Derbylite, 634. Derivation of forms, 52. Descloizite, 514, 648. Description of minerals, 281545. Descriptive terms, 265. Determination tables, 595. Deweylite, 671, 678. Diadochite, 654. Diallage, 424. Diamond, 281, 682. Diaphorite, 624. Diaspore, 365, 680. Diatomaceous earth, 371. Dichroism, 189, Dichroite, 440. Dichroscope, 190. Dickinsonite, 653. Didigonal axis, 14- equatorial, 113. polar, 117. Dietrichite, 636. Dietzeite, 663. Digonal axis, 14- equatorial, 121. holoaxial, 118. polar, 124. Dihexagonal alternating, 89. axis, 14- equatorial, 85. hemipyramid, 92. polar, 92. pyramid, 86. prism, 87. Dihydrite, 644. Dimetasilicic acid, 234. Dimorphism, 220. Diopside, 423, 677. Dioptase, 99, 452, 644. Diorthosilicate, 233. Dioxides, 347. Diploid, 59. Directional properties, 3. Disluite, 650. Dispersion, of light, 166. of optic axes, 206. of the bisectrix, 207. crossed, 208. horizontal, 208. inclined, 208. Disthene, 462. Distortion, 28. Ditesseral central, 47- polar, 64. Ditetragonal alternating, 69. axes, 14. equatorial, 65. hemipyramid, 72. polar, 71. prism, 67. pyramid, 66. Ditrigonal axis, 14. equatorial, 101. hemipyramid, 103. polar, 103. prism, 102. pyramid, 101. Dodecahedron, pentagonal, 60. rhombic, 52. Dog-tooth spar, 381. Dolerophane, 642. Dolomite, 99, 387, 602, 660. Dome, 113, 115. Domeykite, 621. Double refraction, 168. Drawing of crystals, 81. Drusy, 268. Ductility, 258. Dufrenite, 510, 515, 654. Dufrenoysite, 622. Dull luster, 279. Dumortierite, 681. Durangite, 653. Dyscrasite, 625. Dysluite, 377. Ecdemite, 646. Eclogite, 445. Edingtonite, 479, 671. Effloresce, 225. Eglestonite, 641. Eisenrosen, 344. Elaeolite, 440. Elastic, 257. Electron, 291, 631. Elements, 220, 232, 281. table of, 223. Ellipsoid, 184. 688 INDEX Ellipsoid, Fletcher, 185. of revolution, 184- Embolite, 330, 645. Emerald, 436. Emery, 343. Emplectite, 320, 628. Enantiomorphous, 27. Enargite, 326. Endlichite, 511. Enstatite, 421, 604, 613, 677, 681. Epiboulangerite, 624. Epididymite, 674,*676. Epidote, 464, 466, 605, 616, 674, 676. Epigenite, 621. Epistilbite, 479, 671. Epsomite, 119, 538, 600, 638. Equatorial, 126. didigonal, 113. digonal, 121. dihexagonal, 85. ditetragonal, 65. ditrigonal, 101. hexagonal, 93. tetragonal, 72. trigonal, 105. Erbium, tests, 572. Erinite, 643. Erythrite, 517, 651. Etch figures, 62, 78. Ettringite, 662. Eucairite, 627. Euchroite, 643. Euclase, 680. Eucryptite, 441. Eudialyite, 665. Eudidymite, 674, 676. Eulytite, 651. . Eutectics, 241. Euxenite, 668. Evansite, 518, 656. Excelsior, diamond, 283. Expansion, 3. Extinction, 186. angles, 185, 198. inclined, 198. parallel, 187. straight, 187. Extraordinary ray, 169. Faces, similar, 11. vicinal, 267. Fairfieldite, 655. False topaz, 359. Famatinite, 623. Faujasite, 673. Fayalite, 450, 658. Feldsite, 408. Feldspar group, 403, 404. lime, 413. Feldspar soda, 411. potash, 403. Feldspars, monoclinic, 403. triclinic, 411. Feldspathoids, 439. Felsobanyite, 677. Ferberite, 542. Fergusonite, 506, 634, 669. Ferronatrite, 635. Ferrites, 233, 337. Fibroferrite, 636. Fibrolite, 461. Fibrous, 266. Fillowite, 653. First median line, 173. First order colors, 184. Fischerite, 656. Flame, 548. Flame coloration, 553. Fletcher ellipsoid, 185. Flinkite, 652. Flint, 361. Florencite, 656. Flos ferri, 393. Fluellite, 679. Fluocerite, 678. Fluor-apatite, 509. Fluorescence, 280. Fluorine, tests, 589. Fluorite, 331, 603, 609, 662. Fluorspar, 331, 609, 662. Foliated, 269. Fontainebleau, 380. Footeite, 644. Forbesite, 651. Forms, 21. derivation, 25. enantiomorphic, 27. fundamental, 23. gyroidal, 26. hemihedral, 25. holohedral, 24. holosymmetric, 23. polar, 55.. tetartohedral, 27. Formula, empirical, 227. general, 231. molecular, 2. structural, 227. Forsterite, 450, 680. Fowlerite, 430. Fracture, 256, 258. Franckeite, 624. Franklinite, 375, 596, 633. Freezing point, 1. Freibergite, 325. Freieslebenite, 624. Friable, 258. Friedelite, 666. Fuchsite, 490. Fusibility, 555. INDEX 689 G Gadolinite, 681. Gageite, 340. Gahnite, 377, 650. Galena, 298, 627. Galenite, 298, 597, 627. Galenobismuthite, 320, 628. Gallium, tests, 577. Ganomalite, 648. Ganophyllite, 666. Garnet, 442, 605, 610. composition, 231. group, 442. precious, 443. Garnierite, 500, 608. Gaylussite, 401, 661. Gearksutite, 663. Gehlenite, 681. Gelatinization, 591. Geniculate twins, 141. Genthite, 500, 669. Geocronite, 624. Geodes, 270. Gerhardtite, 645. Germanium, tests, 586. Gersdorffite, 316, 597, 622. Geyserite, 370. Ghost, 276. Gibbsite, 365, 656, 678. Gismondite, 479, 671. Glass, 610. Glassware, 556. Glassy, 279. Glauberite, 528, 662. Glaucochroite, 665. Glaucodote, 319, 621. Glaucophane, 676. Gmelinite, 479, 671. Goethite, 658. Gold, 291, 582, 631. Goldschmidtite, 626. Goniometer, 11. contact, 12, 149. reflecting, 12, 150. signal, 152. two-circle, 150. use of, 149. Goslarite, 539, 639. Gossan, 253. Gothite, 363, 633. Goyazite, 656. Granular structure, 268. Graphite, 284, 596, 597, 609, 634. Gravity, specific, 260. Greasy luster, 279. Greenockite, 93, 306, 651. Grossularite, 443, 676. Ground water, 251. Griinlingite, 626. Guanajuatite, 627. 2r Guarinite, 667. Guitermanite, 622. Gummite, 526. Gypsum, 536, 601, 613, 662. Gyrolite, 671. H Habit, 29. Hackly fracture, 258. Haidingerite, 653. Halite, 79, 327, 600, 639. Haloids, 233, 327. Halotrichite, 636. Hamlinite, 655. Hancockite, 648. Hanksite, 89, 535, 637. Hardness, 258, 259. Hardystonite, 649. Hambergite, 664. Harmotome, 482, 670, 672. Hatchettolite, 668. Hauchecornite, 625. Hauerite, 631. Hausmannite, 632. Hatiyne, 438, 674. Haiiynite, 438, 674. Heat conductivity, 3. of fusion, 239. Heating, in closed tube, 557. in open tube, 558. on charcoal, 551. Heavy liquids, 263, 264. Heavy spar, 528. Hedenbergite, 424. Heintzite, 664. Helvite, 666. Hemafibrite, 652. Hematite, 91, 343, 596, 598, 607, 609, 634, 657. Hematolite, 652. Hemidomes, 117. Hemihedrism, diagonal-faced, 26. gyroidal, 26. parallel-faced, 26. Hemimorphism, 25. Hemimorphite, 118, 472. Hemiorthodome, 127. pyramid, 117. prism, 126. Hercynite, 371, 682. Herderite, 655. Herrengrundite, 642. Hessite, 626. Hessonite, 443. Heulandite, 481, 603, 670. Hexagonal alternating, 97. axes, 85. axial ratio, 112. axis, 14. equatorial, 93. hemipyramid, 1st order, 92. 690 INDEX Hexagonal hemipyramid, 2d order, hemipyramid, 3d order, 100. holoaxial, 95. holosymmetric, 85. polar, 99. prism, 1st order, 88. 2d order, 88. 3d order, 94. pyramid 1st order, 86. 2d order, 87. 3d order, 94. system, 17, 84. trapezohedron, 96 Hexahedron, 61. Hexagonite, 433. Hexoctahedron, 48. Hextetrahedron, 55. Hiddenite, 427. Hielmite, 635. Hiortdahlite, 675. Hisingerite, 659. Hoernesite, 653. Holoaxial, 61. digonal, 118. hexagonal, 95. tesseral, 61. tetragonal, 75. trigonal, 108. Holohedral, 24. Holosymmetric, 24- cubic, 47. hexagonal, 85. monoclinic, 121. orthorhombic, 113. tetragonal, 65. triclinic, ISO. Homilite, 664. Hope diamond, 282. Hopeite, 650. Horizontal dispersion, 208. Hornblende, 433, 676. Horn silver, 330. Horseflesh ore, 310. Hortonolite, 658. Howlite, 664. Hubnerite, 542, 665. Humite, 471, 679. Hyacinth, 458. Hyalite, 370. Hyalotekite, 648. Hydrargillite, 365. Hydroboracite, 664. Hydrocerussite, 647. Hydrochloric acid, 560. Hydrocyanite, 641. Hydrofluoric acid. 233. Hydrogen sulphide, 232. Hydrogiobertite, 661. Hydroherderite, 655. Hydromagnesite, 362, 661. Hydronephelite, 479, 673. Hydrophilite, 639. Hydrotalcite, 663. Hydroxides, 362. Hydrozincite, 649. Hypersthene, 421, 604, 616, 682. Ice, 337. Iceland spar, 384. Icositetrahedron, 61. Idocrase, 456. Ihleite, 636. Ilesite, 636. Ilmenite, 346, 609, 633. Ilvaite, 472, 633, 658. Inclined dispersion, 208. Inclusions, 9. Index of refraction, 165, 217. determination of, 209. Indicatrix, 185, 197. Indices, 18. determination of, 156. law of rational, 20. of refraction, 165, 209. Indicolite, 475. Indium, tests, 577. Inesite, 666. Intercepts, 18. Interference, 181. colors, 183. Interference figure, biaxial, 200. uniaxial, 191. Intergrowths of minerals, 243. Interpenetration twins, 141* Intumescence, 555. Iodine, 589. lodobromite, 645. lodyrite, 330, 646. lolite, 440, 613, 680. Iridescence, 183, 274. Iridium, 632. Iridium, tests, 582. Iridosmine, 632. Iron, 631. Iron cap, 253. Iron, tests for, 575. native, 293. specular, 344. titaniferous, 346. Irregularity of crystals, 29. Isoclasite, 656. Isometric system, 16, 47. Isomorphism, 228. Isomorphous groups, 229. Isomorphous mixture, 229. Isotropic crystals, 163. Itacolumite, 282. Jacobsite, 371, 633. Jacynth, 458. INDEX 691 Jade, 426. Jadeite, 420, 426, 676. Jamesonite, 320, 624. Jargon, 458. Jarosite, 635. Jasper, 361. Jeffersonite, 424, 650. Jeremejevite, 665. Jordanite, 622. Josephinite, 632. Joly's balance, 261. K Kainite, 534, 638. Kalgoorlite, 626. Kalinite, 542, 600, 638. Kaliophilite, 441. Kallilite, 625. Kammererite, 669. Kaolin, 501. ( Kaolinite, 501, 601, 613, 678. Keilhauite, 668. Kentrolite, 627, 648. Kermesite, 640. Kieserite, 335, 539, 638. Kilbrickenite, 624. Klaprotholite, 628. Knebelite, 451, 658. Knoxvillite, 636. Kobellite, 625. Koh-i-noor diamond, 282. Koninckite, 655. Kornerupine, 681. Krennerite, 626, Krohnkite, 641. Kunzite, 427. Labradorite, 416, 604, 674. Lamellar, 269. Lampadite, 369. Langbeinite, 638. Langite, 642. Lanthanum, tests, 572. Lapis-lazuli, 439, 608< Larkinite, 652. Laumontite, 479, 671. Laurionite, 647. Laurite, 287. Lautarite, 663. Lautite, 621. Law, of rational indices, 19. Lawsonite, 674. Lazulite, 515, 654, 656. Lazurite, 438, 674. Lead, 293, 628. coat, 580. oxide, 341. tests for, 579. use of test, 559. Leadhillite, 646. Lecontite, 637. Left-handed crystal, 27. Lenarkite, 647. Lengenbachite, 621. Lepidolite, 495, 601, 671. Lepidomelane, 659. Lettering, 33. Leucite, 416, 604, 613, 680. Leucochalcite, 643. Leucophanite, 672. Leucophcenicite, 650. Leucopyrite, 622. Leucoxene, 347. Levynite, 671. Libethenite, 513, 644. Light, 160. interference, 181. convergent, 192. polarized, 175. Light waves, 161. refraction of, 164- Lillianite, 628. Limestone, 385. Limonite, 363, 596, 606, 607, 633, 658. Linarite, 642. Lindackerite, 642. Linnaeite, 631. Lintonite, 488. Liroconite, 643. Lithia mica, 495. Lithium, tests, 564. flux, 559. Livingstonite, 623. Lodestone, 374. Lollingite, 622. Lorandite, 640. Lossenite, 646. Loweite, 539, 637. Lowigite, 677. Ludlamite, 654. Liineburgite, 657. Luster, 273, 278. Lydia stone, 361. M Macro-axis, 113. Macrodome, 115. Macropinacoid, 116. Macroprism, 115. Macropyramid, 114- Magma, 243. Magnesia micas, 497. Magnesium, tests, 567. Magnesite, 386, 602, 661. Magnesoferrite, 634. Magnetism, 552. Magnetite, 373, 596, 609. Malachite, 398, 608, 641. Malacon, 458. Mallardite, 639. Malleability, 258. 692 INDEX Mamillary, 273. Manebach twins, 406. Manganese, tests, 574. Manganite, 367, 596, 632. Mangano-columbite, 665. Manganoferrite, 371. Manganosite, 340, 667. Manganostibnite, 651. Marble, 385. Marcasite, 317, 599, 630. Margarite, 496, 672. Marialite, 453, 676. Marshite, 645. Martinite, 656. Martite, 345. Mascagnite, 638. Massicot, 341, 649. Massive, 268. Matildite, 629. Matrass, 557. Mauzeliite, 646. Mazapilite, 652. Mean refractive index, 171. Measurement of crystals, 149. Meerschaum, 501. Meionite, 453. Melaconite, 340, 596, 631. Melanite, 444. Melanocerite, 665. Melanophlogite, 680. Melanostibian, 633. Melanotekite, 628, 648. Melanterite, 539, 600, 636. Melilite, 675. Meionite, 626. Melting point, 238. Menaccanite, 346, 596. Mendeleef's table, 223. Mendozite, 637. Meneghinite, 624. Mercury, 293, 631. Mercury, tests, 579. Mesitite, 229, 388. Mesolite, 673. Metacinnabar, 305. Metallic luster, 274. Metallic mirror, 557. Metamorphism, 248. Metasilicates, 233. Metasilicic acid, 233. Metastable, 243. ' Metavoltaite, 635. Methylene iodide, 211, 263. Miargyrite, 625. Mica, 488. brittle, 496. group, 488. percussion figure, 489. plate, 194. 1st class, 493. 2d class, 493. Micaceous, 257. cleavage, 489. hematite, 343. Microcline, 409, 604, 613, 675. Microlite, 669. Microperthite, 408. Microsommite, 674. Miersite, 646. Milarite, 675. Millerian indices, 18. Millerite, 307, 599, 630. Mimetite, 95, 512, 646. Mineral, 219. groups, 232. Mineralizers, 246. Minerals, color of, 273. index of refraction, 165. luster of, 274, 278. rock-forming, 609. specific gravity, 260. table of, 617. Minervite, 656. Minium, 649. Mirabilite, 535, 600, 637. Mispickel, 318. Mixite, 642. Mizzonite, 453. Models, 28. Moh's scale of hardness, 259. Molecular, volume, 228. Molybdates, 236. Molybdenite, 296, 596, 597, 630. Molybdenum, tests, 586. Monazite, 507, 603, 657. Monetite, 655. Monobromonaphthalene, 215. Monochromatic light, 183. Monoclinic crystals, 120. axial ratio, 128. dispersion, 207. extinction, 198. pyroxenes, 423. system, 17, 120. twins, 148. Monoxides, 337. Montanite, 651. Montebrasite, 657. Monticellite, 449. Montroydite, 641. Mordenite, 479, 672. Morenosite, 539, 637, 669. Morganite, 437. Mosandrite, 668, Moss agate, 361. Mossite, 634. Muscovite, 488, 489, 601, 614, 672. N Nadorite, 647. Nagyagite, 625. INDEX 693 Nailhead spar, 379. Nantokite, 644. Nasonite, 648. Native elements, 281. Natrolite, 486, 603, 673. Natron, 400, 639. Natrophilite, 653. Naumannite, 627. Negative birefringence, 169. crystals, 169, 170. forms, 27, 79. Nemalite, 362. Neodynium, 572. Nepheline, 440, 604, 675. Nephelite, 100, 440, 604, 611, 675. Neptunite, 505, 633, 666, 668. Nesquehonite, 661. Niccolite, 309, 599, 622. Nickel, tests, 574. Nicols prism, 179. Niobium (columbium), 570. Niter, 521, 600, 639. Nitrates, 507. tests, 590. Nitric acid, 560. Nitrobarite, 640. Nitrocalcite, 521. Nodular, 269. Non-metallic luster, 274. Nordenskioldine, 646. Normal, 43. salt, 227. Northupite, 661. Nosean, 438. Noselite, 438, 674. Nugget, 269. O Obtuse bisectrix, 173. Ocher, 365. Ochrolite, 647. Octahedrite, 351, 668. Octahedron, 52. hex-, 49. tetragonal tris-, 50. trigonal tris-, 50. Oldhamite, 662. Oligoclase, 415. Olivenite, 513, 643 Olivine, 446, 605, 614, 680. Odors, 552, 557. Okenite, 671, 673. Onyx, 360. Oolitic, 271. Opal, 369, 604, 609, 680. Opaque, 275. Open tubes, 558. Optic axes, 172, 198. axial angle, 17$. measurement of, 205. Optical characters, 174. orientation, 174, 175. properties, 160, 174, 175. sign, 169, 194, 204. Ordinary ray, 169. Organic compounds, 591. Oriental amethyst, 342. emerald, 342. topaz, 342. Origin of minerals, 237. Orloff diamond, 282. Orochlorite, 497. Orofrite, 627. Orpiment, 295, 606, 640. Orthoaxis, 120. Orthoclase, 128, 404, 604, 613, 675. Orthodome, 128. Orthopinacoid, 123. Orthoprism, 123. Orthopyramids, 122. Orthorhombic system, 17, 113. Orthosalts, 233. Orthosilicates, 233, 438. Orthosilicic acid, 233. Orthorhombic amphiboles, 431. axial ratio, 120. crystals, 113. dispersion, 207. hemihedral, 118. hemipyramids, 117. holoaxial, 118. holohedral, 113. holosymmetric, 113. prisms, 115. pryoxene, 419. Oscillatory combinations, 137. Osmium, tests, 582. Ottrelite, 497. Oxidation, 251. Oxides, 233, 337. Oxidizing flame, 549. Pachnolite, 334, 663. Palladium, 632. tests, 582. Pandermite, 525. Paragonite, 672. Parallel-faced hemihedrons, Parallel growths, 134. Paramelaconite, 631. Parameters, 17, 156. topic, 228. Parametral face, 23. Parisite, 661. Parting, 257. Partschinite, 666. Pearceite, 620. Pearly luster, 279. 694 INDEX Pectolite, 437, 603, 673. Peganite, 518, 656. Penfieldite, 647. Pentagonal dodecahedron, 60. didodecahedron, 61. Pentlandite, 308, 630. Percussion figure, 489. Percylite, 644. Periclase, 340, 663. Pericline, twins, 410. Peridote, 446. Periodic table, 323. Perovskite, 505, 634, 668. Perpurite, 654. Perthite, 408. Petalite, 675. Petzite, 626. Phantoms, 376. Pharmacosiderite, 652. Phase, 338. Phenacite, 99, 453, 682. Phenocrysts, 216. Phillipsite, 479, 673. Phlogopite, 494, 601, 672. Phoenicochroite, 649. Phosgenite, 647. Phosphates, 336, 507. Phosphorescence, 379. Phosphorus, tests, 590. Phosphosiderite, 510, 654. Phosphuranylite, 655. Physical properties, 356. Picotite, 373. Picromerite, 539, 638. Piedmontite, 467, 666. Pinacoid, 68. basal, 116, 123. brachy-, 116. cleavage, 356. clino-, 123. macro-, 116. ortho-, 123. Pinakiolite, 664. Pinnoite, 664. Pirssonite, 661. Pisanite, 540, 641. Pisolitic, 371. Pitchblende, 535. Pitt diamond, 383. Pitticite, 652. Placer mining, 391. Plagioclase, 411, 614. Plagiohedral, 26. Plagionite, 624. Plane, axial, 173. basal, 68. composition, 138. diametral, 16. of polarization, 176. Plane of symmetry, 13. of vibration, 176. Plane, parametral, 23. twinning, 138. Planoferrite, 658. Plaster of Paris, 537. Platinum, 387, 631. forceps, 554. tests, 582. wire, 553. Plattnerite, 628, 649. Pleochroism, 189. Plumbago, 384. Plumbojarosite, 647. Pneumatolysis, 348. Point-system, 5. Polar, 55. axis, 55. digonal, 124- didigonal, 117. dihexagonal, 92. ditesseral, 64. ditetragonal, 71. ditrigonal, 103. hexagonal, 99. tesseral, 62. tetragonal, 78. trigonal, 110. Polarization, of light, 175. circular, 176, 196. Polarizer, 177. Pole of face, 43. Polianite, 632. Pollucite, 679. Polyadelphite, 444. Polyargyrite, 336, 625. Polybasite, 336, 624. Polycrase, 635, 668. Polydymite, 307, 630. Polyhalite, 662. Polylithionite, 496. Polymignite, 635. Polymorphism, 319. Polysynthetic twins, 142. Positive birefringence, 169, 170. Positive forms, 27. Potash alum, 543. Potash feldspar, 403. Potash mica, 489. Potassium mercuric iodide, 363. Potassium, tests, 563. Powdery, 369. Powellite, 544, 670. Praseodymium, tests, 572. Precious garnet, 443. opal, 369. Prehnite, 470, 604, 676. Pressure figure, 490. 3 riceite, 535. 3 rimary optic' axes, 172. 3 rimary minerals, 344. rimitive circle, 43. Prism, 113. INDEX 695 Prism, brachy, lid. dihexagonal, 87. ditetragonal, 67. ditrigonal, 102. macro, 115. nicol, 179. ortho, 123. tetragonal, 67. trigonal, 108. 1st order tetragonal, 67. 2d order hexagonal, 88. 3d order, hexagonal, 94- 2d order tetragonal, 68. Prismatic cleavage, 256. Prismatic habit, 265. Prochlorite, 678. Projection, 31. axes, 35. clinographic, 31, 33. orthographic, 31. stereographic, 42. Prolectite, 679. Prosopite, 663, 679. Proustite, 323, 598, 607, 645. Pseudobrookite, 634. Pseudomalachite, 644. Pseudomorph, 361, 363. Pseudosymmetry, 141. Psilomelane, 368, 596, 632. Ptiolite, 479. Pucherite, 651. Pycnometer, 263. Pyramid, 113. dihexagonal, 86. ditrigonal, 101. herni-, 71. hexagonal, 86. monoclinic, 122. tetragonal, 66. trigonal, 102. Pyrargyrite, 322, 596, 607, 624, 645. Pyrite, 313, 599, 609, 630. arsenical, 318. class, 313. copper, 310. group, 313. magnetic, 308. twins, 141. Pyritohedron, 60. Pyroaurite, 658. Pyrochlore, 668. Pyrochroite, 362, 632, 667. Pyroelectric, 475. Pyrolusite, 352, 596, 597, 632. Pyromorphite, 95, 511, 602, 606, 648. Pyrope, 443, 677. Pyrophane, 347. Pyrophanite, 632. Pyrophillite, 502, 678. Pyrosmaltite, 659. Pyrostilpnite, 645. Pyroxene, 604, 608, 615, 677. group, 419, 420. monoclinic, 423. orthorhombic, 419. Pyrrhotite, 308, 599, 609, 630. Q Quartz, 110, 352, 605, 611, 681. interference figure, 196. rotary polarization, 197. smoky, 359. wedge, 182, 184, 187, 196. Quartzite, 356. Quenstedtite, 636. Quicksilver, 293. R Radiated, 269. Radicles, 224. Radium, 514, 567. Raimondite, 637. Ralstonite, 679. Rammelsbergite, 316, 622. Ranite, 479. Raspite, 649. Rational indices, 19. Ray, extraordinary. 169. ordinary, 169. Realgar, 294, 606, 640. Reagents, 559, 560. Reddingite, 655. Reducing flame, 550. Reduction with soda, 579. Reentrant angle, 134. Reflected light, 163. Reflection goniometer, 12, 150. Reflection twins, 140. Refraction, 164. double, 168. indices of, 165. mean index of, 171. ( Refractometer, 212, 213. Regent diamond, 282. Reinite, 633. Remingtonite, 669. Rensslerite, 500. Repeated twinning, 142. Replacement, 22. Resinous luster, 279. Reticulated, 269. Rhabdophanite, 657. Rhagite, 650. Rhodium, 582. Rhodizite, 663. Rhodochrosite, 391, 602, 667. Rhodonite, 132, 430, 604. Rhombic dodecahedron, 52. Rhombohedral carbonates, 379. 696 INDEX Rhombohedron, 90. Richardite, 626. Hichterite, 667. Kirbeckite, 435, 659. Right-handed crystals, 27, 197. Rinkite, 667. Ripidolite, 497. Roasting, 552. Rock crystal, 358. Rock salt, 327. Rock sections, 216. Roeblingite, 647. Romerite, 540, 636. Roepperite, 451. Rontgen ray, 280. Roscoelite, 490, 670. Rotation twins, 137. Rotatory polarization, 196. Rubellite, 475. Rubidium, tests, 564. Ruby, 342. silver, 322, 323. spinel, 371. Ruthenium, 582. Rutile, 349, 597, 598, 605, 612, 634, 668 S Safflorite, 315, 621. Sagenite, 350. Salt, 240. Saltpeter, 521. Salts, 224. acid, 226. basic, 226. hydrated, 227. normal, 226. Samarium, 572. ' Sand, black, 288. green, Sandstone, 356. Sanidine, 408. Sapphire, 342. Sarcolite, 675. Sartorite, 320, 622. Sassolite, 523, 640. Satin spar, 537. Scale of colors, 184. of fusibility, 555. of hardness, 260. Scalenohedron, 90. , tetragonal, 60, 70. Scandium, 572. Scapolite, 453, 612. Schapbachite, 628. Scheelite, 543, 603, 668. Schefferite, 424, 666. Schiller, 422. Schirmerite, 628. Schorl, 474. Schorlomite, 444, 667. Schwartzenbergite, 648. Schwartzite, 325. Scleromater, 259. Scolecite, 479, 673. Scorodite, 652. Second order of colors, 184' Secondary enrichment, 253. Secondary minerals, 244. Secondary optic axes, 172. Sectility, 258. Selenite, 537. Selenium, 537, 587. Sellaite, 677. Senaite, 627. Senarmontite, 346, 640. Sensitive plate, 199. Sepiolite, 501, 601, 678. Sericite, 491. Serpentine, 498, 601, 614, 678. Sesquioxides, 341. Siderite, 91, 388, 602, 606, 657. Sideronatrite, 635. Sign of birefringence, 169, 173. Silicates, 233, 403. classification of, 233. Silicic acids, 233. salts of, 233. Silicon, tests, 591. Silicified wood, 361. Silky luster, 279. Sillimanite, 461, 614, 681. Silver, 290, 578, 597, 631. glance, 297. Similar faces, 11. Sipylite, 669. Skutterudite, 315, 621. Smaltite, 315, 598, 621. Smarskite, 634. Smithsonite, 392, 603, 649. Smoky quartz, 359. Snow, 337. Soapstone, 500. Soda, 558. Sodalite, 438, 604, 610, 675. Sodalite group, 438. Soda niter, 520, 600, 639. . Sodium, tests for, 564. Solutions, 248. Space-lattice, 4. Spadaite, 671. Spangolite, 642. Spathic iron ore, 388. Specific gravity, 261. heat, 239. Specular iron ore, 344. Sperrylite, 287, 623. Spessartite, 444, 666. Sphaerite, 518, 656. Sphaerocobaltite, 393, 669. Sphalerite, 301, 596, 602, 606, 607, 630, 649. INDEX 697 Sphene, 503, 616. Sphenoid, 70. orthorhombic, 118. Spinel, 371, 605, 610, 682. group, 371. twins, 143. Splintery, 358. Spodumene, 426, 605, 675. Stalactite, 273. Stalagmite, 273. Stannite, 312, 629. Star of the south diamond, 282. Stassfurtite, 522. Staurolite, 477, 605, 616, 680. Steatite, 500. Stelznerite, 642. Stephanite, 325, 597. Stercorite, 656. Stereographic projection, 42. Sternbergite, 629. Stibnite, 295, 597, 623. Stilbite, 483, 602, 670. Stilpnomelane, 498, 659. Stokesite, 646. Stolzite, 545, 649. Straight extinction, 187. Streak, 274. Stream tin, 349. Strengite, 510, 654. Striations, 137, 266. Stromeyerite, 301, 629. Strontianite, 396, 602, 660. Structural formula, 227. Struvite, 118, 656. Stylotypite, 623. Subconchoidal fracture, 258. Sublimates, 7, 557. Sulpharsenates, 323. Sulphates, 236. Sulphides, 232, 294. Sulpho acids, 232, 320. Sulphohalite, 638. Sulphur, 285, 587, 601, 606, 640. Sulvanite, 629. Supplementary forms, 27. twins, 140. Surfaces of crystals, 266. Sussexite, 664. Svabite, 653. Svanbergite, 655. Swallow-tail twins, 536. Sylvanite, 626. Sylvite, 328, 600, 639. Symmetry, 13. alternating, 15. axes of, 13, 15. center of, 13. planes of, 13. pseudo-, 146. Synadelphite, 652. Syngenite, 638, 662. System, cubic, 47. hexagonal, 84. monoclinic, 120. orthorhombic, 123. tetragonal, 65. triclinic, 129. two-component, 240. Szaibelyite, 664. Table, for blowpipe determination, 617. of rock-forming minerals, 609. for determination of common miner- als, 596. . of coats on coal, 593. for fusibility, 555. Tabular habit, 265. Tachydrite, 639. Tagilite, 644. Talc, 500, 601, 614, 663, 679. Tantalates, 236. Tantalite, 506, 635. Tantalum, 571. Tapalpite, 626. Tapiolite, 635. Tarnish, 275. Taste, 618. Tavistokite, 655. Taylorite, 638. Tellurates, 236. Tellurium, 587, 626. Tenacity, 258. Tennantite, 324, 621. Tenorite, 340. Tephroite, 451, 665. Terlinguaite, 641. Tesseral central, 59. holoaxial, 61. polar, 62. Tetartohedral forms, 27. Tetradymite, 625. Tetragonal alternating, 76. axial ratio, 82. axis, 14- equatorial, 72. hemihedral, 72. holoaxial, 75, holohedral, 65. holosymmetric, 65. polar, 78. prism, 67. 1st order, 67. 2d order, 68. 3d order, 74. pyramid, 66. 1st order, 66. 2d order, 67. 3d order, 73. scalenohedron, 69. 698 INDEX Tetragonal system, 16, 65. trapezohedron, 75. trisoctahedron, 50. tristetrahedron, 57. Tetrahedrite, 324, 596, 597. Tetrahedron, 67. tetragonal tris-, 57. trigonal tris-, 56. hex-, 65. Tetrahexahedron, 49. Thallium, tests, 577. Thaumasite, 661. Thenardite, 527, 638. Thermonatrite, 639. Thin sections colors, 216. Third order of colors, 184- Thirty-two types of crystals, 6, 47. Thomsenolite, 663. Thomsonite, 487, 673. Thorite, 457, 679, 681. Thorium, tests, 572. Thoulet's solution, 263. Thulite, 465. Thuringite, 658. Tiemannite, 627. Tiger-eye, 435. Tilasite, 653. Tile ore, 338. Tin, 584, 632. Tincal, 523. Tinstone, 348. Titanates, 233. Titanic iron ore, 346. Titanite, 503, 603, 616, 667. Titanium, tests, 569. oxides, 351. Topaz, 458, 605, 681. false, 359. oriental, 342. Topazolite, 444. Topic parameters, 228. Torbernite, 519, 644. Torrensite, 667. Total reflection, 167. reflectometer, 211. Touchstone, 361. Tough, 257, 258. Tourmaline, 105, 473, 605, 612, 064. sections of, 177. tongs, 178. Translucency, 275. Transparent, 275. Trapezohedron, hexagonal, 96. tetragonal, 75. trigonal, 109. Tremolite, 431, 677. Trichalcite, 643. Triclinic system, 17, 129. Tridymite, 361, 611, 681. Trigonal axis, 14, equatorial, 105. Trigonal holoaxial, 108. polar, 110. prisms, 103, 106. pyramids, 102, 106. trapezohedron, 109. trisoctahedron, 50. tristetrahedron, 56. Trimerite, 666. Trimorphism, 220. Triphylite, 653. Triplite, 654. Triploidite, 654. Tripolite, 371. Tripuhyite, 651. Trisilicates, 234. Trisilicic acid, 234. Tritomite, Trogerite, 652. Troilite, 308, 630. Trona, 401, 600, 639. Troostite, 452. Truncation, 22. Tscheffkinite, 667. Tschermigite, 638. Tungstates, 236. Tungsten, 587. Tungstite, 668. Turgite, 363, 633, 658. Turingite, 498. Turkey fat, 307, 392. Turner's flux, 559. Turquoise, 3, 510, 518, 604. Twin axis, 138. Brazilian, 146. crystals, 138. interpenetrating, 141. lamellae, 142. plane, 138. reflection, 140. rotation, 138. spinel, 143. striae, 142. supplementary, 140. Twinning, 138. lamellae, 142, polysynthetic, 142. Twins, 137. Tychite, 661. Tyrolite, 643. Tysonite, 679. U Ulexite, 524, 601, 663. Ullmannite, 316, 625. Umangite, 627. Uniaxial crystals, 170, 184. interference figure, 191. Unit form, 23. Uralite, 435. Uralitization, 426. INDEX 699 Uranates, 236, 507. Uraninite, 525, 596, 608, 634. Uranium, tests, 576. Uranocircite, 655. Uranophane, 669. Uranophilite, 669. Uranospinite, 653. Uranothallite, 661. Utahite, 636. Uvarovite, 445, 669. Valentinite, 346, 641. Vanadates, 236. Vanadinite, 95, 512, 648. Vanadium, tests, 576. Variscite, 510, 519, 656. Vauquelite, 644. Verdi antique, 499. Vermiculite, 670. Vesuvianite, 455, 605, 611, 676. Veszelyite, 642. Vibrations plane, 176. Vicinal faces, 267. Viluite, 455. Vitreous luster, 279. Vivianite, 510, 516, 608, 654. Volatilization, 552. Volborthite, 645. Voltaite, 636. W Wad, 368, 667. Wagnerite, 657. Walpurgite, 650. Warwickite, 665. Water, 225, 240, 337, 557. of constitution, 226. of crystallization, 225. Wave direction, 161. front, 161. length, 162. surface, 169. Wavellite, 510, 518, 602, 656. Waves of light, 161. Waxy luster, 279. Wellsite, 479, 670. Wernerite, 453, 603, 674, 676. Westphal balance, 264. Whattevillite, 662. Wheel ore, 321. Whitneyite, 621. Willemite, 99, 451, 603, 649. Witherite, 395, 602, 660. Wittichenite, 628. Wohlerite, 669. Wolfachite, 622. Wolfram (Tungsten), 586. Wolframite, 542, 596, 633, 656 Wolframum, tests, 587. Wollastonite, 429, 603, 675. Wood opal, 370. Wulfenite, 78, 545, 602, 648. Wurtzite, 93, 303. Xanthoconite, 645. Xanthophillite, 678. Xanthosiderite, 658. Xenotime, 657. Ytterbium, 572. Yttrium, tests, 572. Yttrocerite, 678. Yttrotantalite, 635. Zaratite, 669. Zeolites, 478, 479, 613. Zepharovichite, 656. Zeunerite, 520, 643. Zinc, 631. tests, 573. Zinc blende, 301. Zincite, 339, 606, 650. Zinkenite, 320. Zinnwaldite, 488, 496. Zircon, 456, 605, 612, 682. Zirconium, tests, 571. Zirkelite, 668. Zoisite, 465, 674, 676. Zone, 22. axis, 22. circle, !$ law, 22. Zunyite, 680. r I A HE following pages contain advertisements of books of kindred interest, or on related subjects. , Economic Geology WITH SPECIAL REFERENCE TO THE UNITED STATES By HEINRICH RIES, A.M., Ph.D., Professor of Economic Geology at Cornell University Third Edition, enlarged and thoroughly revised, 589 pages 237 illustrations, 56 plates, $3.50 net-; by mail, $3.70 " Altogether the work is an admirable one, and we strongly commend it to teachers in this country as a source of concise, accurate, and recent information regarding the mineral deposits of the United States." Nature, London. " All general introductory geological or mineralogical matter, the reader is sup- posed to have acquired. For less important matter slightly smaller type is used. The style is condensed to the last degree, but not at the expense of its clearness, which is French. The result is a compact and excellent book one that every broad-minded business man should have, and that deserves the wide acceptance which it is finding." Science. " Necessarily condensed, it yet covers the ground in a thorough and authoritative manner and will be used by many as the most satisfactory textbook available." H. V. W. in The American Geologist. " The author is to be congratulated on the broad perspective he has of his theme, and the clearness of his style in presenting it. He uses no unnecessary words in his treatise ; he omits none that are requisite to its complete presentation. " It is to the economic phase of geological study that he addresses himself. What the commercial value and uses of the various deposits in the earth's crusts are, 'he tells us in the plainest and most forcible way. He does not entirely avoid other features of geology which have been presented in many other volumes, but he holds himself to the one purpose of showing the industrial and commercial value of clays, and coals, and marbles, and metallic ores. To all those who are interested in mines, and in manufacturing- what mines produce, his work cannot fail to be. of the highest value. " The book is divided into two sections : the first dealing with ' Non-Metallic Minerals' such as coals, petroleum, building stones, cements, gypsum, and others; and the second part treating of 'Metallic Minerals or Ores' such as iron, copper, lead, zinc, aluminum, and many others. The ground covered by the author is very comprehensive and thorough. ''The illustrations and diagrams are numerous and illuminative. The author has had access to plates and cuts of the United States Geological Survey in many in- stances, and has made use of the statistical tables from the same source. Taken all together, the volume is among the choicest of its kind, and we predict for it a wide circulation." New England Journal of Education. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York Economic Geology of the United States WITH BRIEFER MENTION OF FOREIGN MINERAL PRODUCTS By RALPH S. TARR, B.S., F.G.S.A., Assistant Professor of Geology at Cornell University Second Edition Revised $ 3.50 net COMMENTS " I am more than pleased with your new ' Economic Geology of the United States.' An introduction to this subject, fully abreast of its recent progress, and especially adapted to American students and readers, has been a desideratum. The book is admirably suited for class use, and I shall adopt it as the text-book for instruction in Economic Geology in Colorado College. It is essentially accurate, while written in a pleasant and popular style, and is one of the few books on practical geology that the general public is sure to pronounce readable. The large share of attention given to non-metallic resources is an especially valuable feature." FRANCIS W. CRAGIN, Professor of Geology, Mineralogy, and Paleontology at Colorado College. " I have examined Professor R. S. Tarr's ' Economic Geology ' with much pleasure. It fills a felt want. It will be found not only very helpful to students and teachers by furnishing the fundamental facts of the science, but it places within easy reach of the business man, the capitalist, and the statesman, fresh, reliable, and complete statistics of our national resources. The numerous tables bringing out in an analytic way the comparative resources and productiveness of our country and of different states, are a specially convenient and admirable feature. The work is an interesting demonstration of the great public importance of the science of geology." JAMES E. TODD, State Geologist, South Dakota. "It is one of those books that is valuable for what it omits, and for the concise method of presenting its data. The American engineer has now the ability to acquire the latest knowledge of the theories, locations, and statistics of the leading American ore bodies at a glance. Were my course one of text-books, I should certainly use it, and I have already called the attention of my students to its value as a book of reference." EDWARD H. WILLIAMS, Professor of Mining, Engineering, and Geology at Lehigh University. " I have taken time for a careful examination of the work ; and it gives me pleasure to say that it is very satisfactory. Regarded simply as a general treatise on Economic Geology, it is a distinct advance on anything that we had before ; while in its relations to the Economic deposits of this country it is almost a new creation and certainly supplies a want long and keenly felt by both teachers and general students. Its appearance was most timely in my case, and my class in Economic Geology are already using it as a text-book." WILLIAM O. CROSBY, Assistant Professor of Structural and Economic Geology at the Massachusetts Institute of Technology. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York The Natural History of Igneous Rocks By ALFRED BARKER Illustrated, cloth, 8vo, $j.0o net A BRIEF SUMMARY OF CONTENTS Igneous Action in Relation to Geology Vulcanicity Igneous Intrusion Petrographical Provinces Mutual Relations of Associated Igneous Rocks Igneous Rocks and Their Constituents Rock Magmas Crystallization of Rock Magmas Supersaturation and Deferred Crystallization Isomor- phism and Mixed Crystals Structures of Igneous Rocks Mineralizers and Pneumatolysis Magmatic Differentiation Hybridism in Igneous Rocks Classification of Igneous Rocks. Rocks, Rock-weathering, and Soils BY GEORGE P. MERRILL Curator of Department of Geology, United States National Museum, and Professor of Geology in the Corcoran Scientific School, etc. With many Illustrations Full-page Plates and Figures in the Text Second Edition Cloth 8vo Price $4.00 net " This is one of the most useful and most satisfactory manuals that has ap- peared in recent years, possessing as much interest for the geographer as for the geologist." Bulletin Amer. Geog. Society. " In treatment, as in subject, Professor Merrill's work is notable. It is strictly up to date, embracing the results of the latest researches, and duly recognizing the work of contemporary investigators ; also it is made admi- rable mechanically by clear typography, good paper, excellent illustrations, and a full index." National Geographic Magazine. "A book brimful of facts obtained by workers in divers fields. The work forms a highly important addition to our practical knowledge of geology." - Scientific American. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York An Introduction to Geology BY WILLIAM B. SCOTT Illustrated, doth, i2mo, 2.60 net An introduction to the science of geology for both students who desire to pursue the subject exhaustively and those who wish merely to obtain an outline of the methods and principal results of the science. This is not one of the text-books which always pronounces a definite and final opinion. The author holds that in no science are there more open questions than in geology ; in none are changes of view more frequent ; and in none is it more important to emphasize the distinction between fact and inference. The student is here en- couraged to weigh evidence and balance possibilities and to suspend judgment when the testimony is insufficient to justify decision. The author is an advocate of the new geology, and his book presents all the latest advances in the science. The book is very fully illustrated, many of the plates being from photographs taken by the United States Geological Survey. " I have* looked the book through with increasing pleasure. The latest advances in American geology have been taken advantage of, so that the book is up to date. American instructors have been waiting a long time for a book which could be used satisfactorily as a guide in an opening course in geology. Professor Scott's book seems to me to be admirably adapted for this purpose." Professor C. R. VAN HISE, University of Wisconsin. "Professor Scott's Geology seems to me excellently fitted for my beginners at Smith College, and I shall try it there next year. It is a fine book." Professor B. K. EMERSON, Amherst College. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York " THIS BOOK TAKES HIGH RANK AMONG THE FEW GREAT BOOKS UPON GLACIERS.' Characteristics of Existing Glaciers BY WILLIAM HERBERT HOBBS Professor of Geology, University of Michigan Illustrated, cloth, 8vo, $3.25 net ; by mail, " The author has done good service to the glaciologist and glacial geologist in bringing together his concise description and classification of existing glaciers and ice-sheets in the present convenient form. Especially in the parts devoted to Arctic and Antarctic ice he has made an exhaustive digest of the scattered literature, and has presented a copiously illustrated summary of the available information respecting the distribution and character of the ice of these regions. To the end of each chapter he appends a full list of his authorities, so that the book is in every respect a most useful work of reference. . . . Every geographer and geologist interested in ice will appreciate these clear descriptions and excellent illustrations of the earth's great glaciers they make up into a most presentable book." Nature. BY THE SAME AUTHOR Earth Features and their Meaning Profusely Illustrated, 8w, $j.oo net "The book is an excellent reference volume for students who are interested in a simple outline of geology. The volume has been tested in class work and should prove its worth." Bulletin of American Geographical Society. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York RETURN EARTH SCIENCES LIBRARY TO ^ 230 McCone Hall 642-2997 LOAN PERIOD 1 1 MONTH 2 3 4 5 6 ALL BOOKS MAY BE RECALLED AFTER 7 DAYS Books needed for class reserve are subject to immediate recall DUE AS STAMPED BELOW FORM NO. DD8 UNIVERSITY OF CALIFORNIA, BERKELEY BERKELEY, CA 94720 U.C. BERKELEY LIBRARIES, C03MU3LM15